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/lsp/util.lua

@@ -8,6 +8,7 @@ local uv = vim.uv
 local M = {}
 
 --- @param border string|(string|[string,string])[]
+--- @return never
 local function border_error(border)
   error(
     string.format(
@@ -69,8 +70,7 @@ local function get_border_size(opts)
       -- border specified as a list of border characters
       return e
     end
-    --- @diagnostic disable-next-line:missing-return
-    border_error(border)
+    return border_error(border)
   end
 
   --- @param e string
@@ -179,7 +179,7 @@ function M.apply_text_edits(text_edits, bufnr, position_encoding, change_annotat
   local function apply_text_edits()
     -- Fix reversed range and indexing each text_edits
     for index, text_edit in ipairs(text_edits) do
-      --- @cast text_edit lsp.TextEdit|{_index: integer}
+      --- @cast text_edit lsp.TextEdit & { _index?: integer }
       -- XXX: Preserve existing _index to avoid surprises if the same edit is reapplied. #39344
       if text_edit._index == nil then
         text_edit._index = index
@@ -668,7 +668,11 @@ function M.convert_signature_help_to_markdown_lines(signature_help, ft, triggers
   if active_signature >= #signature_help.signatures or active_signature < 0 then
     active_signature = 0
   end
-  local signature = vim.deepcopy(signature_help.signatures[active_signature + 1])
+  local signature = signature_help.signatures[active_signature + 1]
+  if not signature then
+    return
+  end
+  signature = vim.deepcopy(signature)
   local label = signature.label
   if ft then
     -- wrap inside a code block for proper rendering
@@ -1410,7 +1414,7 @@ function M._make_floating_popup_size(contents, opts)
   local title_length = 0
   local chunks = type(opts.title) == 'string' and { { opts.title } } or opts.title or {}
   for _, chunk in
-    ipairs(chunks --[=[@as [string, string][]]=])
+    ipairs(chunks --[=[@as [string, string][] ]=])
   do
     title_length = title_length + vim.fn.strdisplaywidth(chunk[1])
   end