Add ability to detect the GDScript language

A fairly new project on GitHub at
https://github.com/git-learning-game/oh-my-git

shows the primary language to be GDScript at 94.6%

Turns out that it is a recently emerging language used primarily by game engines:
https://gdscript.com/

The Scan results for this package downloaded from
https://github.com/git-learning-game/oh-my-git/archive/refs/tags/0.6.4.tar.gz

returned a language of "TypeScript" but that is not really correct. We should add the ability to detect GDScript code. A quick browse of the project in question shows that the file extension for a GDScript source file is .gd.