feat(lua)!: execute Lua with "nvim -l"

Problem:
Nvim has Lua but the "nvim" CLI can't easily be used to execute Lua
scripts, especially scripts that take arguments or produce output.

Solution:
- support "nvim -l [args...]" for running scripts. closes #15749
- exit without +q
- remove lua2dox_filter
- remove Doxyfile. This wasn't used anyway, because the doxygen config
  is inlined in gen_vimdoc.py (`Doxyfile` variable).
- use "nvim -l" in docs-gen CI job

Examples:

    $ nvim -l scripts/lua2dox.lua --help
    Lua2DoX (0.2 20130128)
    ...

    $ echo "print(vim.inspect(_G.arg))" | nvim -l - --arg1 --arg2
    $ echo 'print(vim.inspect(vim.api.nvim_buf_get_text(1,0,0,-1,-1,{})))' | nvim +"put ='text'" -l -

TODO?
  -e executes Lua code
  -l loads a module
  -i enters REPL _after running the other arguments_.

runtime/doc/lua.txt

@@ -21,15 +21,19 @@ Nvim includes a "standard library" |lua-stdlib| for Lua.  It complements the
 which can be used from Lua code (|lua-vimscript| |vim.api|). Together these
 "namespaces" form the Nvim programming interface.
 
-See the |lua-guide| for an introduction to using Lua in Neovim.
+Lua plugins and user config are automatically discovered and loaded, just like
+Vimscript. See |lua-guide| for practical guidance.
 
+You can also run Lua scripts from your shell using the |-l| argument: >
+    nvim -l foo.lua [args...]
+<
                                                                   *lua-compat*
 Lua 5.1 is the permanent interface for Nvim Lua.  Plugins need only consider
 Lua 5.1, not worry about forward-compatibility with future Lua versions.  If
 Nvim ever ships with Lua 5.4+, a Lua 5.1 compatibility shim will be provided
 so that old plugins continue to work transparently.
 
-------------------------------------------------------------------------------
+==============================================================================
 LUA CONCEPTS AND IDIOMS                                         *lua-concepts*
 
 Lua is very simple: this means that, while there are some quirks, once you
@@ -73,25 +77,24 @@ In Lua, any missing arguments are passed as `nil`. Example: >lua
 Furthermore it is not an error if extra parameters are passed, they are just
 discarded.
 
-It is also allowed to omit the parentheses (only) if the function takes
-exactly one string (`"foo"`) or table literal (`{1,2,3}`). The latter is often
-used to approximate the "named parameters" feature of languages like Python
-("kwargs" or "keyword args"). Example: >lua
+                                                                      *kwargs*
+When calling a function, you can omit the parentheses if the function takes
+exactly one string literal (`"foo"`) or table literal (`{1,2,3}`). The latter
+is often used to approximate "named parameters" ("kwargs" or "keyword args")
+as in languages like Python and C#. Example: >lua
     local func_with_opts = function(opts)
         local will_do_foo = opts.foo
         local filename = opts.filename
-
         ...
     end
 
     func_with_opts { foo = true, filename = "hello.world" }
 <
-There is nothing special going on here except that parentheses are treated as
+There's nothing special going on here except that parentheses are treated as
 whitespace. But visually, this small bit of sugar gets reasonably close to
 a "keyword args" interface.
 
 It is of course also valid to call the function with parentheses: >lua
-
     func_with_opts({ foo = true, filename = "hello.world" })
 <
 Nvim tends to prefer the keyword args style.
@@ -100,25 +103,20 @@ Nvim tends to prefer the keyword args style.
 LUA PATTERNS                                                    *lua-patterns*
 
 Lua intentionally does not support regular expressions, instead it has limited
-"patterns" which avoid the performance pitfalls of extended regex.
-|luaref-patterns|
+"patterns" |luaref-patterns| which avoid the performance pitfalls of extended
+regex. Lua scripts can also use Vim regex via |vim.regex()|.
 
-Examples using |string.match()|: >lua
+These examples use |string.match()| to demonstrate Lua patterns: >lua
 
     print(string.match("foo123bar123", "%d+"))
     -- 123
-
     print(string.match("foo123bar123", "[^%d]+"))
     -- foo
-
     print(string.match("foo123bar123", "[abc]+"))
     -- ba
-
     print(string.match("foo.bar", "%.bar"))
     -- .bar
 
-For more complex matching you can use Vim regex from Lua via |vim.regex()|.
-
 ==============================================================================
 IMPORTING LUA MODULES                                  *require()* *lua-require*
 
@@ -389,7 +387,7 @@ For example consider the following Lua omnifunc handler: >lua
     vim.api.nvim_buf_set_option(0, 'omnifunc', 'v:lua.mymod.omnifunc')
 
 Note: The module ("mymod" in the above example) must either be a Lua global,
-or use the require syntax as specified above to access it from a package.
+or use require() as shown above to access it from a package.
 
 Note: `v:lua` without a call is not allowed in a Vimscript expression:
 |Funcref|s cannot represent Lua functions. The following are errors: >vim

runtime/doc/news.txt

@@ -57,6 +57,11 @@ The following new APIs or features were added.
 <
   (or the Vimscript equivalent) to their |config| file.
 
+• Run Lua scripts from your shell using |-l|. >
+      nvim -l foo.lua --arg1 --arg2
+<  Also works with stdin: >
+      echo "print(42)" | nvim -l -
+
 • Added a |vim.lsp.codelens.clear()| function to clear codelenses.
 
 • |vim.inspect_pos()|, |vim.show_pos()| and |:Inspect| allow a user to get or show items

runtime/doc/starting.txt

@@ -9,7 +9,7 @@ Starting Vim						*starting*
                                       Type |gO| to see the table of contents.
 
 ==============================================================================
-Nvim arguments						*vim-arguments*
+Nvim arguments						*cli-arguments*
 
 Most often, Nvim is started to edit a single file with the command: >
 
@@ -31,8 +31,8 @@ filename	One or more file names.  The first one will be the current
 		To avoid a file name starting with a '-' being interpreted as
 		an option, precede the arglist with "--", e.g.: >
 			nvim -- -filename
-<		All arguments after the "--" will be interpreted as file names,
-		no other options or "+command" argument can follow.
+<		All arguments after "--" are interpreted as file names, no
+		other options or "+command" argument can follow.
 
 							*--*
 `-`		Alias for stdin (standard input).
@@ -143,15 +143,13 @@ argument.
 		these commands, independently from "-c" commands.
 
 							*-S*
--S {file}	Vimscript or Lua (".lua") {file} will be |:source|d after the
-		first file has been read. Equivalent to: >
+-S [file]	Vimscript or Lua (".lua") [file] will be |:source|d after the
+		first file has been read or "Session.vim" if [file] is not
+		given. Equivalent to: >
 			-c "source {file}"
 <		Can be repeated like "-c", subject to the same limit of 10
 		"-c" arguments. {file} cannot start with a "-".
 
--S		Works like "-S Session.vim".  Only when used as the last
-		argument or when another "-" option follows.
-
 -L							*-L* *-r*
 -r		Recovery mode.  Without a file name argument, a list of
 		existing swap files is given.  With a file name, a swap file
@@ -211,10 +209,30 @@ argument.
 			nvim -es +":verbose echo 'foo'"
 			nvim -V1 -es +foo
 
-<		User |config| is skipped (unless given with |-u|).
+<		User |config| is skipped unless |-u| was given.
 		Swap file is skipped (like |-n|).
 		User |shada| is loaded (unless "-i NONE" is given).
 
+							*-l*
+-l {script} [args]
+		Executes Lua {script} file and exits. All [args] (up to "--"
+		|---|) are treated as {script} args, not Nvim args: by Lua
+		convention they are set in the `_G.arg` global table.  *lua-args*
+		On {script} error, Nvim exits with code 1.
+
+		Sets 'verbose' to 1 (like "-V1"), so Lua `print()` writes to
+		output.
+
+		Any |cli-arguments| before "-l" are processed before executing
+		{script}. For example this quits before executing "foo.lua": >
+			nvim +q -l foo.lua
+<		This loads Lua module "bar" before executing "foo.lua": >
+			nvim +"lua require('bar')" -l foo.lua
+<
+		User |config| is skipped unless |-u| was given.
+		User |shada| is skipped unless |-i| was given.
+		Swap file is skipped (like |-n|).
+
 							*-b*
 -b		Binary mode.  File I/O will only recognize <NL> to separate
 		lines.  The 'expandtab' option will be reset.  The 'textwidth'
@@ -222,9 +240,6 @@ argument.
 		is set.  This is done after reading the |vimrc| but before
 		reading any file in the arglist.  See also |edit-binary|.
 
-							*-l*
--l		Lisp mode.  Sets the 'lisp' and 'showmatch' options on.
-
 							*-A*
 -A		Arabic mode.  Sets the 'arabic' option on.
 
@@ -239,10 +254,10 @@ argument.
 		Example: >
 			nvim -V8
 
--V[N]{filename}
-		Like -V and set 'verbosefile' to {filename}.  Messages are not
-		displayed; instead they are written to the file {filename}.
-		{filename} must not start with a digit.
+-V[N]{file}
+		Like -V and sets 'verbosefile' to {file} (must not start with
+		a digit).  Messages are not displayed; instead they are
+		written to {file}.
 		Example: >
 			nvim -V20vimlog
 <