2D physics in XZ plane

[Nahor]

It would be great if bevy_xpdb_2d could handle the 2D<->3D conversions using the XZ-plane instead of just XY-plane.

The existing behavior (bevy_xpdb 0.4.2) works well for pure 2D games and for pseudo-3D (2D world using 3D models) side-view games. It's not so great for pseudo-3D top-down games, where the plane is XZ, with rotation around the Y axis.

(to be clear, I do not think a Position should be a "(x,z)" vector, I think it should remain a typical 2D "(x,y)" vector, but that vector should map to the x and z coordinates of a Vector3)