Change most nodes to simply inherit from Node2D like SimplePolygon2D

[9thAzure]

The primary reason why this isn't the case was because I didn't know how to deal with concave shapes (hence, inheritance from Polygon2D), but now I know about Geometry2D. decompose_polygon_in_convex

With this, the only other benefit I see with Polygon2D is the storage of the shape in scene, such that the shape doesn't have to entirely regenerated when the scene is run. I think to add this back in though.

Also, SimplePolygon2D was originally created to keep this original design. Since RegularPolygon2D and StarPolygon2D are to be reverted back to this original design, and CollisionPolygon2D has already merged the regular and star shape generation, I see no reason not to merge all of them into one.