Merge #268

268: godot-core: builtin: reimplement Plane functions/methods r=Bromeon a=T4rmin

Partly addresses issue #209 by re-implementing `Plane`'s functions/methods.

I know there's a need to implement test cases for it by looking at the `Rect2i` commits, but I would like to know if this part is correct yet since I am an still an absolute beginner at this stuff. Please let me know of anything that is an issue.

Also, would implementing test cases for `Plane` be similar in implementation as the `Rect2i` ones?

Co-authored-by: Aaron Lawrence <t4rmin@protonmail.com>