refactor!: reorganize bones-bevy-renderer.

[zicklag]

Submitting on behalf of @tekhaeraav:

This is a reorganization of bones-bevy-renderer that cleans up some dead code and attempts to make the renderer easier to understand without making any breaking changes.

[zicklag]

This needs to be tested with Jumpy to make sure there's no regressions, but otherwise I approve ✅.

There's one added feature that lets you start the game without waiting for assets to load, in case you want to handle asset loading yourself ( a use-case in the Bones game maker UI @TekhnaeRaav is working on ).

The rest is minimally changed functionally, mostly re-organized.

[MaxCWhitehead]

Looks great thank you @TekhnaeRaav!

[MaxCWhitehead]

Once get CI errors cleaned up + confirm works on jumpy can merge.

[MaxCWhitehead]

Seems to work with jumpy - just had to define the preload setting.