Move from Codemirror to Monaco

[Atulin]

Since there's no way to edit code inside of an external code editor, perhaps it's time to bring a proper code editor to Greasemonkey instead? Muhammad, mountain, the whole thing.

CodeMirror, as it's implemented in GM at least, provides nothing more than syntax highlighting and comment/uncomment shortcut. There's zero autocompletion, zero linting, zero... anything. That's fine for simple scripts, but as they get bigger it would be nice to have an editor that's more than a glorified textarea.