2D support

[AlCappaccino]

Right now I can see this only works for 3d rendering and with 3d physics (bevy_xpbd_3d) would it be possible to make it 3d/2d agnostic?

[mbrea-c]

I'd like to add 2d support, though I'm thinking I might have to use 2d/3d sub-crates like xpbd does (if nothing else, to choose the appropriate xpbd dependency). Better ideas are welcome of course. Support for 2d cameras will also be needed, though I haven't worked with 2d in Bevy yet so I'm not sure how much it diverges from the 3d side.

This is not needed in my projects at the moment, but I get that it's a blocker for many so I'll try to work something out at some point after the 0.13 migration.

[artokun]

2d would be nice.