GDScript syntax highlighting

[Bromeon]

It would be nice to have ```gdscript code tags.
We currently use ```php for some, which works somewhat due to overlap in keywords, but it's not ideal 😀

The mdBook docs contain some more information on the topic; we would likely need our own highlight.js.

[snatvb]

I found https://github.com/highlightjs/highlightjs-gdscript
Need to try
Maybe I'll try later to integrate it