Add EntityWorldMut::(try_)resource_scope

[BigWingBeat]

Objective

Fixes #20139

Solution

Implement the methods, and leverage them where applicable

Testing

Added unit tests

---

Showcase

let id = entity_world_mut.id();
let world = entity_world_mut.into_world_mut();
world.resource_scope::<_, _>(|world, res| {
    let entity_world_mut = world.entity_mut(id);
    /* ... */
});

becomes

entity_world_mut.resource_scope::<_, _>(|entity, res| {
    /* ... */
});

[alice-i-cecile]

I don't quite see how this change was necessary for this PR? That said it's a good change, so I'm happy to keep it :)

[alice-i-cecile]

The changes to this file similarly seem unrelated, but I also like the cleanup.

[alice-i-cecile]

Very solid implementation, tested, documented and something that we definitely want.

I don't love tackling cleanup in the same PR, but I won't look a gift horse in the mouth. The cleanup is vaguely related, and well done; might as well merge it.

My only blocking complaint is that I feel strongly that the TODOs in bevy_transform should not be done; we should remove them.

[BigWingBeat]

I don't love tackling cleanup in the same PR, but I won't look a gift horse in the mouth. The cleanup is vaguely related, and well done; might as well merge it.

Yeah, it's just easy opportunistic stuff I noticed while looking at the _scope APIs. I suppose it's just somewhat of a habit from solo-dev projects, where PR hygiene isn't as important. I'll keep this in mind for the future.