I was thinking, it would be pretty neat if we could edit Haxe code from GMEdit, and by pressing a hotkey, have it compile to GML and move to the GMS project in the same editor.

🤔

It would certainly be a lovely feature, and you can find a very basic highlighter for Haxe here (used for hscript embedded in Markdown), but I would not expect anyone to take up this task, as it is maybe few hundred hours of work to adequately handle Haxe syntax highlighting and auto-completion, and I can't think of a good reason why anyone would want to do that to themselves (given that there's plenty of existing editors with Haxe support)