build: replace LuaLS with EmmyLua

Problem: LuaLS struggles with the generics used in Nvim's runtime,
requiring broad diagnostic suppressions. Indexing is also slow.

Solution: Use EmmyLua for type checks in the build and CI. It offers
more sophisticated type checking, substantially better support for
generics, and much better flow analysis.

Correct the affected annotations. Use `@internal`, supported directly by
EmmyLua, instead of `@nodoc` for shared internal declarations, and
support it in the help parser.

AI-assisted

runtime/lua/vim/fs.lua

@@ -173,7 +173,7 @@ end
 ---
 ---@since 15
 ---@param path string Filepath (or other identity string).
----@param opts? table
+---@param opts? { maxlen?: integer } #
 ---  - maxlen: (integer, default: 180) Max length (bytes) of the result.
 ---@return string # Filesystem-safe, mnemonic slug.
 function M.slug(path, opts)