Add PANDA3D_physics_collision_shapes

[Moguri]

Finally getting around to re-introducing a new physics extension(s) based off of the BLENDER_physics extension (#1104). Some discussion points:

• What is the best why to note previous contributions? I am currently the only contributor for this new extension, but it is based heavily on the old BLENDER_physics extension which had other contributors at the time. Perhaps @stevenvergenz and @Kupoman can comment on how they would prefer to be credited for their contributions?
• Is there a better scope than "physics"? This scope works for me, but I would like to get others' feedback.

Note that this only covers collision shapes from the old BLENDER_physics extension. Rigid body physics information (e.g., mass, damping) is left for another extension.

Also, while this PR lists known implementations, those implementations are still a work in progress.