bevy_scene: Add ReflectBundle

[Shatur]

Objective

Similar to #6344, but contains only ReflectBundle changes. Useful for scripting. The implementation has also been updated to look exactly like ReflectComponent.

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Changelog

Added

• Reflection for bundles.

[MrGVSV]

Note: My approval should probably be taken with a grain of salt as this code is based on #6344 (not trying to intentionally dupe the reviewer system lol)

[nicopap]

There is a few issues with this code:

1. how much the implementation duplicates ReflectComponent
2. how many failure cases we have. A lot of panic!.
3. The indentation is all over the place.

Performance

I did take a look at alternative implementation. Notably: could we use DynamicBundle::get_components to avoid having to recursively navigate the struct? I think yes, but it's not relevant for this PR.

Things to do for approval

The fn defined in ReflectBundleFns are very large, so it's going to bloat binaries, pretty bad for wasm.

All said. I think it's fine as of this PR, since you have to explicitly register the ReflectBundle, so no one is paying for things they don't use, at the cost of some convenience (the need to add a register_data::<Bundle, ReflectBundle>()). So I'm approving if the following are done:

1. Replace the if let … else x with let else x; … to avoid this rightward drift
2. Remove the documentation, replace them with reference to doc items in reflect/component.rs

[Shatur]

@nicopap done!
Should I also link fn_pointers and new to the one from ReflectComponent?

[nicopap]

I think the docs are pretty good at this point.

[github-actions]

Example alien_cake_addict failed to run, please try running it locally and check the result.

[github-actions]

Example alien_cake_addict failed to run, please try running it locally and check the result.