Add ReflectKind

[doonv]

Objective

Fix #11657

Solution

Add a ReflectKind enum, add Reflect::reflect_kind which returns a ReflectKind, and add kind method implementions to ReflectRef, ReflectMut, and ReflectOwned, which returns a ReflectKind.

I also changed AccessError to use this new struct instead of it's own TypeKind struct.

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Changelog

• Added ReflectKind, an enumeration over the kinds of a reflected type without its data.
• Added Reflect::reflect_kind (with default implementation)
• Added implementation for the kind method on ReflectRef, ReflectMut, and ReflectOwned which gives their kind without any information, as a ReflectKind

[alice-i-cecile]

I like this: there's clearly a need for this information, as shown by our own duplicated error type.

That said, this is technically breaking, and needs a migration guide.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[doonv]

While yes this is technically breaking compared to main, it's not breaking when compared to 0.12, so I don't think a migration guide is necessary.

[pablo-lua]

This changes are great and can very easily improve the code quality with reflection in the future.

[alice-i-cecile]

@pablo-lua, if you're comfortable, feel free to leave a full Approval :) Like we talk about in CONTRIBUTING.md, community approvals are really vital for Bevy!

[pablo-lua]

Great, will do that :)