add support for Godot's gd-scripts
Add support for .gd scripts used by the Godot game engine. https://godotengine.org
Language description: https://docs.godotengine.org/en/stable/getting_started/scripting/gdscript/gdscript_basics.html
The basic structure of the xml is taken from the python.xml since both languages share a lot of syntactic features.
One open issue is that the .gd file extension used by Godot is also used by the GAP. Since I assume Godot is currently the more actively used language I took the liberty to set the priority of this highlighter to 5. How are such issues solved usually?
Documentation of GAP's file extensions https://www.gap-system.org/Manuals/doc/ref/chap83.html