docs: add lua typing for vim.NIL

runtime/lua/vim/_meta/builtin.lua

@@ -1,7 +1,8 @@
 ---@meta
-
 -- luacheck: no unused args
 
+error('Cannot require a meta file')
+
 ---@defgroup vim.builtin
 ---
 ---@brief <pre>help
@@ -62,6 +63,12 @@
 ---
 ---</pre>
 
+---@class vim.NIL
+
+---@type vim.NIL
+---@nodoc
+vim.NIL = ...
+
 --- Returns true if the code is executing as part of a "fast" event handler,
 --- where most of the API is disabled. These are low-level events (e.g.
 --- |lua-loop-callbacks|) which can be invoked whenever Nvim polls for input.