Flush commands after every mutation in `WorldEntityMut`

[nakedible]

Objective

• Currently adding observers spawns an entity which implicitly flushes the command queue, which can cause undefined behaviour if the WorldEntityMut is used after this
• The reason WorldEntityMut attempted to (unsuccessfully) avoid flushing commands until finished was that such commands may move or despawn the entity being referenced, invalidating the cached location.
• With the introduction of hooks and observers, this isn't sensible anymore as running the commands generated by hooks immediately is required to maintain correct ordering of operations and to not expose the world in an inconsistent state
• Objective is to make command flushing deterministic and fix the related issues
• Fixes #16212
• Fixes #14621
• Fixes #16034

Solution

• Allow WorldEntityMut to exist even when it refers to a despawned entity by allowing EntityLocation to be marked invalid
• Add checks to all methods to panic if trying to access a despawned entity
• Flush command queue after every operation that might trigger hooks or observers
• Update entity location always after flushing command queue

Testing

• Added test cases for currently broken behaviour
• Added test cases that flushes happen in all operations
• Added test cases to ensure hooks and commands are run exactly in correct order when nested

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Todo:

• [x] Write migration guide
• [x] Add tests that using EntityWorldMut on a despawned entity panics
• [x] Add tests that commands are flushed after every operation that is supposed to flush them
• [x] Add tests that hooks, observers and their spawned commands are run in the correct order when nested

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Migration Guide

Previously EntityWorldMut triggered command queue flushes in unpredictable places, which could interfere with hooks and observers. Now the command queue is flushed always immediately after any call in EntityWorldMut that spawns or despawns an entity, or adds, removes or replaces a component. This means hooks and observers will run their commands in the correct order.

As a side effect, there is a possibility that a hook or observer could despawn the entity that is being referred to by EntityWorldMut. This could already currently happen if an observer was added while keeping an EntityWorldMut referece and would cause unsound behaviour. If the entity has been despawned, calling any methods which require the entity location will panic. This matches the behaviour that Commands will panic if called on an already despawned entity. In the extremely rare case where taking a new EntityWorldMut reference or otherwise restructuring the code so that this case does not happen is not possible, there's a new is_despawned method that can be used to check if the referred entity has been despawned.