gdformat: support raw string literals (r-strings)

[Arrow-x]

Formatting this

func test():
	var g = r"\,? ?(i?\(.*?\))"

will through a massive Exception (should I post the full log?)

AttributeError: 'PatternStr' object has no attribute 'raw'

it seems that it doesn't recognize r-string that was added in 74995

[Scony]

Correct, I've found out about Godot's support for raw strings yesterday - from release notes. It's not supported yet.