GDScript syntax highlighting

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](https://rust-lang.github.io/mdBook/format/theme/syntax-highlighting.html) contain some more information on the topic; we would likely need our own `highlight.js`.

Start with the linked mdBook syntax-highlighting documentation, then inspect the book's current PHP code tags. Done means GDScript code blocks can use ```gdscript and receive appropriate highlighting instead of relying on PHP keyword overlap.

Written by the indexing model from the issue text.