AnimationEvents should also be EntityEvents

## What problem does this solve or what need does it fill?

This lets observers of AnimationEvnets be entity-local instead of global.

## What solution would you like?

Deriving AnimationEvent should also impl EntityEvent, with the target entity being `event.trigger().animation_player`, which is what the migration guide said to use in lieu of `event.target()`.

## What alternative(s) have you considered?

Using markers to use distinguish entities in global observers.

Also, trying to derive both AnimationEvent and EntityEvent on one struct gives an error about conflicting implementations of Event.

## Additional context

The migration guide implies this should be possible:
```rs
#[derive(AnimationEvent)]
struct SayMessage(String);

animation.add_event(0.2, SayMessage("hello".to_string()));
world.entity_mut(animation_player).observe(|say_message: On| {
println!("played on", say_message.trigger().animation_player);
})
```
However, when using `commands.spawn(...).observe(...)`, it errors that the AnimationEvent isn't an EntityEvent and can't be used for entity observers.

Start with the migration guide example and the AnimationEvent and EntityEvent derive entry points, then trace how commands.spawn(...).observe(...) validates observer event types. Done means an AnimationEvent can be used with an entity observer whose target is event.trigger().animation_player, without conflicting Event implementations.

Written by the indexing model from the issue text.