Implement a CodeHighlighter for .yarn scripts

[dogboydog]

Not sure the priority of this, but if you add "yarn" to the "Text File Extensions" setting in EditorSettings, you can go to File > Open from the Script panel and open a yarn file. It would be interesting to try to have syntax highlighting in the Godot Editor. However personally I think the best experience will probably still be using the VS Code extension.

https://docs.godotengine.org/en/stable/classes/class_codehighlighter.html

Depending on the implementation, it might make sense to make the CodeHighlighter its own plugin.

[dogboydog]

I wonder if the language server used by the VS Code plugin could be useful to implement this.