Add Support for GDScript Syntax Highlighting

[ghost]

GDScript is the official scripting language of the promising Godot game engine.

It is rising in popularity and becoming a force on its own for game development.

Supporting it for syntax highlighting makes total sense imo.

[fxha]

We use CodeMirror to display code blocks. Unfortunately GDScript syntax highliting is not available, you can find a list of all languages here. Maybe you or someone else can provide/create a custom syntax file.

[Jocs]

@infosisio We have change the highlight library from CodeMirror to Prism, but it still not support GDScript you can make a PR to prism add GDScript highlight support.