Add rotated_around for Vector2, Vector3 to GDScript and C#

[profan]

Related to: godotengine/godot-proposals#235

Implements helper functions to rotate a vector around a specific origin to Vector2 and Vector3.

Additions

• Vector2::rotated_around, which rotates a vector around a specific origin, by an angle.
• Vector3::rotated_around_axis, which rotates a vector around a specific origin given a rotation axis and angle.
• Vector3::rotated_around, which rotates a vector around a specific origin given a rotation represented as a quaternion.

Implemented for GDScript and C#, includes documentation changes and tests.
I think this change should be compatible with 3.x as well, but I'm not 100%.

... I added this mostly because I wanted it for myself, and I figure others might find it useful :)
(and it also happens to be readily available in eg. Unity)

I was considering adding a rotated_around for Vector3 that takes a Quat as well, so it's more generally useful (and that's the version I'd personally implement on my own projects), but maybe that's a bit much? ... Added it after all, seems useful.

[zinob]

I like it! This is doable today, but it would be really neat to not have to reinvent the wheel every time I need to rotate something around something.