Add support for GDScript 2 / Godot 4

[jasonmccollum]

Rouge currently has support for GDScript 1, but is missing support for GDScript 2, which is used in Godot 4.x: https://docs.godotengine.org/en/stable/tutorials/scripting/gdscript/gdscript_basics.html

As the core syntax is largely the same between versions (ie functions and variable declarations are the same between 1 and 2), it might be possible to simply add the additional keywords to the existing GDScript lexer to support both versions of the language with the existing lexer unless there's a preference for having a separate one for each version of the language.