Add @:luaNotLocal compiler metadata and lua_no_xpcall compiler flag for Lua backend

Hi everyone, Thanks for making Haxe! :)

I wish if Haxe's Lua backend would introduce/add these 2 because really good...

1. `@luaNotLocal` can be used to disable generating `local` variables and just let them global, The purpose of this is to let some game engines/tools like [Scrupp](https://scrupp.sourceforge.net) works because it does not allow game loop table to be local variable...

2. `lua_no_xpcall` can be helpful in case `xpcall` does not work properly on Lua interpreter dev working with, In addition this can help making Haxe externs for Lua homebrews much easier, As [MicroLua](https://sourceforge.net/projects/microlua) (3DS/NDS Lua homebrew) does not work with `xpcall` and to get it working i was forced to edit last line of code to call main function with no `xpcall` usage, The same even goes to [Amulet](https://www.amulet.xyz) i think so...

Sorry if this issue not helpful, I ❤️ Haxe and i wish if i can get stuff working with no headache of editing generated code!

Thanks for reading and awaiting replies...