fix(watch): invalid joined path #37973

Problem:
When vim._watch.watch() is used to watch a single file, libuv returns
the basename as the filename argument in the callback. The code joins
this with the watched path, producing a nonsensical path like
"/path/to/file.lua/file.lua", which causes ENOTDIR errors on
subsequent fs_stat calls.

Solution:
Check whether the watched path is a directory before joining the
filename. When watching a file, ignore the filename from libuv and
use the watched path directly.

runtime/lua/vim/_watch.lua

@@ -69,10 +69,12 @@ function M.watch(path, opts, callback)
   local uvflags = opts and opts.uvflags or {}
   local handle = assert(uv.new_fs_event())
 
+  local watching_dir = (uv.fs_stat(path) or {}).type == 'directory'
+
   local _, start_err, start_errname = handle:start(path, uvflags, function(err, filename, events)
     assert(not err, err)
     local fullpath = path
-    if filename then
+    if filename and watching_dir then
       fullpath = vim.fs.normalize(vim.fs.joinpath(fullpath, filename))
     end