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/doc/syntax.txt

@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 23
+*syntax.txt*	For Vim version 9.1.  Last change: 2025 Feb 25
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -2343,13 +2343,9 @@ instead, and the name of your source file should be *.pike
 
 LUA						*lua.vim* *ft-lua-syntax*
 
-The Lua syntax file can be used for versions 4.0, 5.0, 5.1 and 5.2 (5.2 is
-the default). You can select one of these versions using the global variables
-lua_version and lua_subversion. For example, to activate Lua
-5.1 syntax highlighting, set the variables like this: >
-
-	:let lua_version = 5
-	:let lua_subversion = 1
+The Lua syntax file can be used for versions 4.0, 5.0+. You can select one of
+these versions using the global variables |g:lua_version| and
+|g:lua_subversion|.
 
 
 MAIL						*mail.vim* *ft-mail.vim*