runtime(lua): Update lua ftplugin and documentation

Problem:
- The doc says the default `g:lua_subversion` is 2, but in fact it is 3
  (see `runtime/syntax/lua.vim`)
- `includeexpr` doesn't work with module in `init.lua`

Solution:
- Update documentation
- Assign value to option `&include`
- Add function `LuaInclude` and assign it to `l:&includeexpr`

closes: #16655

Co-authored-by: dkearns <dougkearns@gmail.com>
Signed-off-by: brianhuster <phambinhanctb2004@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/lua.vim

@@ -2,7 +2,7 @@
 " Language:     Lua 4.0, Lua 5.0, Lua 5.1, Lua 5.2 and Lua 5.3
 " Maintainer:   Marcus Aurelius Farias <masserahguard-lua 'at' yahoo com>
 " First Author: Carlos Augusto Teixeira Mendes <cmendes 'at' inf puc-rio br>
-" Last Change:  2022 Sep 07
+" Last Change:  2025 Feb 25
 " Options:      lua_version = 4 or 5
 "               lua_subversion = 0 (for 4.0 or 5.0)
 "                               or 1, 2, 3 (for 5.1, 5.2 or 5.3)
@@ -16,6 +16,7 @@ endif
 let s:cpo_save = &cpo
 set cpo&vim
 
+" keep in sync with ftplugin/lua.vim
 if !exists("lua_version")
   " Default is lua 5.3
   let lua_version = 5