Godot add-on: Add support for formatting built-in sripts

[NicholasBolen]

Currently, attempting to format/re-order on a built-in script causes errors.

Would love to see support for this, though I'm unsure how challenging it might be to implement.

[NathanLovato]

It's most likely possible, and probably not too hard given Godot's API. I'll leave it up as something for potential contributors to pick up if they'd like.