Add the ability to cancel delayed commands

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

When using delayed commands for things like animations, sound effects or gameplay effects,
you'll often want to *stop* the command from taking effect if the originating entity dies.

## What solution would you like?

Expose the entity reference of the delayed command tracking entity somehow during delayed command creation.

## What alternative(s) have you considered?

Current workaround: query DelayedCommandQueue, despawn what you want cancelled.

Alternatively, embed the orginating entity in the delayed command, and check during application that the entity still exists.

## Additional context

Noticed when writing the release notes for Bevy 0.19 :)