refactor(gen_lsp.lua): add typing for the LSP protocol JSON data model

Enhance readability and intellisense by incorporating type annotations.
Types are not very strict and may not encompass th entire LSP Protocol
metamodel; the scope is up to what's relevant for generating type
annotations for LSP (`_meta/protocol.lua`).

Based on the model schema:
https://raw.githubusercontent.com/microsoft/language-server-protocol/gh-pages/_specifications/lsp/3.18/metaModel/metaModel.schema.json

No behavioral changes (and hence no diff on _meta/protocol.lua) should
exist in this commit.

scripts/gen_lsp.lua

@@ -24,7 +24,17 @@ local function tofile(fname, text)
   end
 end
 
----@param opt gen_lsp._opt
+--- The LSP protocol JSON data (it's partial, non-exhaustive).
+--- https://raw.githubusercontent.com/microsoft/language-server-protocol/gh-pages/_specifications/lsp/3.18/metaModel/metaModel.schema.json
+--- @class vim._gen_lsp.Protocol
+--- @field requests vim._gen_lsp.Request[]
+--- @field notifications vim._gen_lsp.Notification[]
+--- @field structures vim._gen_lsp.Structure[]
+--- @field enumerations vim._gen_lsp.Enumeration[]
+--- @field typeAliases vim._gen_lsp.TypeAlias[]
+
+---@param opt vim._gen_lsp.opt
+---@return vim._gen_lsp.Protocol
 local function read_json(opt)
   local uri = 'https://raw.githubusercontent.com/microsoft/language-server-protocol/gh-pages/_specifications/lsp/'
     .. opt.version
@@ -46,6 +56,7 @@ local function name(s)
   return s:gsub('^%$', 'dollar'):gsub('/', '_')
 end
 
+---@param protocol vim._gen_lsp.Protocol
 local function gen_methods(protocol)
   local output = {
     '-- Generated by gen_lsp.lua, keep at end of file.',
@@ -56,6 +67,32 @@ local function gen_methods(protocol)
   }
   local indent = (' '):rep(2)
 
+  --- @class vim._gen_lsp.Request
+  --- @field deprecated? string
+  --- @field documentation? string
+  --- @field messageDirection string
+  --- @field method string
+  --- @field params? any
+  --- @field proposed? boolean
+  --- @field registrationMethod? string
+  --- @field registrationOptions? any
+  --- @field since? string
+
+  --- @class vim._gen_lsp.Notification
+  --- @field deprecated? string
+  --- @field documentation? string
+  --- @field errorData? any
+  --- @field messageDirection string
+  --- @field method string
+  --- @field params? any[]
+  --- @field partialResult? any
+  --- @field proposed? boolean
+  --- @field registrationMethod? string
+  --- @field registrationOptions? any
+  --- @field result any
+  --- @field since? string
+
+  ---@type (vim._gen_lsp.Request|vim._gen_lsp.Notification)[]
   local all = vim.list_extend(protocol.requests, protocol.notifications)
   table.sort(all, function(a, b)
     return name(a.method) < name(b.method)
@@ -106,14 +143,15 @@ return protocol
   vim.cmd.write()
 end
 
----@class gen_lsp._opt
+---@class vim._gen_lsp.opt
 ---@field output_file string
 ---@field version string
 ---@field methods boolean
 
----@param opt gen_lsp._opt
+---@param opt vim._gen_lsp.opt
 function M.gen(opt)
-  local protocol = read_json(opt) --- @type table
+  --- @type vim._gen_lsp.Protocol
+  local protocol = read_json(opt)
 
   if opt.methods then
     gen_methods(protocol)
@@ -144,6 +182,7 @@ function M.gen(opt)
 
   local anonymous_num = 0
 
+  ---@type string[]
   local anonym_classes = {}
 
   local simple_types = {
@@ -154,32 +193,65 @@ function M.gen(opt)
     'decimal',
   }
 
+  --- @class vim._gen_lsp.Type
+  --- @field kind string a common field for all Types.
+  --- @field name? string for ReferenceType, BaseType
+  --- @field element? any for ArrayType
+  --- @field items? vim._gen_lsp.Type[] for OrType, AndType
+  --- @field key? vim._gen_lsp.Type for MapType
+  --- @field value? string|vim._gen_lsp.Type for StringLiteralType, MapType, StructureLiteralType
+
+  ---@param type vim._gen_lsp.Type
+  ---@return string
   local function parse_type(type)
+    -- ReferenceType | BaseType
     if type.kind == 'reference' or type.kind == 'base' then
       if vim.tbl_contains(simple_types, type.name) then
         return type.name
       end
       return 'lsp.' .. type.name
+
+    -- ArrayType
     elseif type.kind == 'array' then
       return parse_type(type.element) .. '[]'
+
+    -- OrType
     elseif type.kind == 'or' then
       local val = ''
       for _, item in ipairs(type.items) do
-        val = val .. parse_type(item) .. '|'
+        val = val .. parse_type(item) .. '|' --[[ @as string ]]
       end
       val = val:sub(0, -2)
       return val
+
+    -- StringLiteralType
     elseif type.kind == 'stringLiteral' then
       return '"' .. type.value .. '"'
+
+    -- MapType
     elseif type.kind == 'map' then
-      return 'table<' .. parse_type(type.key) .. ', ' .. parse_type(type.value) .. '>'
+      local key = assert(type.key)
+      local value = type.value --[[ @as vim._gen_lsp.Type ]]
+      return 'table<' .. parse_type(key) .. ', ' .. parse_type(value) .. '>'
+
+    -- StructureLiteralType
     elseif type.kind == 'literal' then
       -- can I use ---@param disabled? {reason: string}
       -- use | to continue the inline class to be able to add docs
       -- https://github.com/LuaLS/lua-language-server/issues/2128
       anonymous_num = anonymous_num + 1
       local anonym = { '---@class anonym' .. anonymous_num }
-      for _, field in ipairs(type.value.properties) do
+
+      --- @class vim._gen_lsp.StructureLiteral translated to anonymous @class.
+      --- @field deprecated? string
+      --- @field description? string
+      --- @field properties vim._gen_lsp.Property[]
+      --- @field proposed? boolean
+      --- @field since? string
+
+      ---@type vim._gen_lsp.StructureLiteral
+      local structural_literal = assert(type.value) --[[ @as vim._gen_lsp.StructureLiteral ]]
+      for _, field in ipairs(structural_literal.properties) do
         if field.documentation then
           field.documentation = field.documentation:gsub('\n', '\n---')
           anonym[#anonym + 1] = '---' .. field.documentation
@@ -195,6 +267,8 @@ function M.gen(opt)
         anonym_classes[#anonym_classes + 1] = line
       end
       return 'anonym' .. anonymous_num
+
+    -- TupleType
     elseif type.kind == 'tuple' then
       local tuple = '{ '
       for i, value in ipairs(type.items) do
@@ -204,10 +278,20 @@ function M.gen(opt)
       tuple = tuple:sub(0, -3)
       return tuple .. ' }'
     end
-    vim.print(type)
+
+    vim.print('WARNING: Unknown type ', type)
     return ''
   end
 
+  --- @class vim._gen_lsp.Structure translated to @class
+  --- @field deprecated? string
+  --- @field documentation? string
+  --- @field extends? { kind: string, name: string }[]
+  --- @field mixins? { kind: string, name: string }[]
+  --- @field name string
+  --- @field properties? vim._gen_lsp.Property[]  members, translated to @field
+  --- @field proposed? boolean
+  --- @field since? string
   for _, structure in ipairs(protocol.structures) do
     if structure.documentation then
       structure.documentation = structure.documentation:gsub('\n', '\n---')
@@ -225,6 +309,15 @@ function M.gen(opt)
     else
       output[#output + 1] = '---@class lsp.' .. structure.name
     end
+
+    --- @class vim._gen_lsp.Property translated to @field
+    --- @field deprecated? string
+    --- @field documentation? string
+    --- @field name string
+    --- @field optional? boolean
+    --- @field proposed? boolean
+    --- @field since? string
+    --- @field type { kind: string, name: string }
     for _, field in ipairs(structure.properties or {}) do
       if field.documentation then
         field.documentation = field.documentation:gsub('\n', '\n---')
@@ -239,6 +332,14 @@ function M.gen(opt)
     output[#output + 1] = ''
   end
 
+  --- @class vim._gen_lsp.Enumeration translated to @enum
+  --- @field deprecated string?
+  --- @field documentation string?
+  --- @field name string?
+  --- @field proposed boolean?
+  --- @field since string?
+  --- @field suportsCustomValues boolean?
+  --- @field values { name: string, value: string, documentation?: string, since?: string }[]
   for _, enum in ipairs(protocol.enumerations) do
     if enum.documentation then
       enum.documentation = enum.documentation:gsub('\n', '\n---')
@@ -256,6 +357,13 @@ function M.gen(opt)
     output[#output + 1] = ''
   end
 
+  --- @class vim._gen_lsp.TypeAlias translated to @alias
+  --- @field deprecated? string?
+  --- @field documentation? string
+  --- @field name string
+  --- @field proposed? boolean
+  --- @field since? string
+  --- @field type vim._gen_lsp.Type
   for _, alias in ipairs(protocol.typeAliases) do
     if alias.documentation then
       alias.documentation = alias.documentation:gsub('\n', '\n---')
@@ -274,6 +382,7 @@ function M.gen(opt)
     output[#output + 1] = ''
   end
 
+  -- anonymous classes
   for _, line in ipairs(anonym_classes) do
     output[#output + 1] = line
   end
@@ -281,6 +390,7 @@ function M.gen(opt)
   tofile(opt.output_file, table.concat(output, '\n'))
 end
 
+---@type vim._gen_lsp.opt
 local opt = {
   output_file = 'runtime/lua/vim/lsp/_meta/protocol.lua',
   version = DEFAULT_LSP_VERSION,