initial work towards ordering schedules within state transition groups

[lee-orr]

Objective

Fixes #13711

Solution

Testing

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Changelog

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  • If you can't summarize the work, your change may be unreasonably large / unrelated. Consider splitting your PR to make it easier to review and merge!

Migration Guide

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• If this PR is a breaking change (relative to the last release of Bevy), describe how a user might need to migrate their code to support these changes
• Simply adding new functionality is not a breaking change.
• Fixing behavior that was definitely a bug, rather than a questionable design choice is not a breaking change.

[alice-i-cecile]

Good, I like this solution! Bother me when this is ready for review please. We'll ship this with 0.14.

[alice-i-cecile]

Closing in favor of #13763.

I am happy with @mini's solution - it feels a bit clearer than mine, and they set up some initial tests & made the necessary doc changes