Migrate to Bevy 0.15

[voximity]

This is a migration to Bevy 0.15. It is simply composed of all necessary changes to make bevy-tnua build with Bevy 0.15, bevy_rapier 0.28, and the tentative Avian 0.2.

There are some problems that currently exist, some likely block this PR:

• Avian has not published the version supporting 0.15 (I think version 0.2.0 is in progress?)
• No consideration/work done to convert Bundle pattern to required components
• Rapier now supports multiple physics "worlds," since RapierConfiguration/RapierContext are now components instead of resources. bevy-tnua may require implementation changes to support this feature properly
• The character does not properly float/move/respond in all Avian demos
  • Might be related to Avian's PhysicsLayer changes - Make PhysicsLayer require a #[default] variant Jondolf/avian#494
  • Did some minimal troubleshooting to not much avail

As per #45, I did not make any assumptions about how to adjust the API for the new Bevy version - I provide this as a starting point, and to anyone who depends on tnua and wants to migrate their project to 0.15.

[idanarye]

Thanks! I'll take it from here (can't promise to publish today, but I'll try to do it during the weekend)

[idanarye]

I looked into the why the floating problem in the Avian demos, and it looks like some of the components were changed. Specifically:

• Something was changed with the layers. Instead of having all the layers, objects created without layers only have the default layer (i.e. the one generated by Default::default())
• Mass and AngularInteria are not automatically added - I have to look in ComputedMass and ComputedAngularInteria instead.