Allow the editing of startup schedules

[ItsDoot]

Objective

Fixes #10968

Solution

Pull startup schedules from a list of ScheduleLabels in the same way the update schedules are handled.

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Changelog

• Added MainScheduleOrder::startup_labels to allow the editing of the startup schedule order.

Migration Guide

• Added a new field to MainScheduleOrder, startup_labels, for editing the startup schedule order.

[alice-i-cecile]

Good! That's about as easy as I hoped it would be.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[alice-i-cecile]

Oh, this is technically breaking since we've added a new field to a public struct. For all of the 0 people explicitly constructing this!

[ItsDoot]

Oh, this is technically breaking since we've added a new field to a public struct. For all of the 0 people explicitly constructing this!

Added a simple migration guide 👍

[andriyDev]

This might be another issue, but should we also add support for PreFixedUpdate/PostFixedUpdate? This same mechanism could be used for that too.

Someone in the discord was asking about PostFixedUpdate 😅

[alice-i-cecile]

That's clever. Happy to see it done, here or elsewhere, but we shouldn't block this PR on that.

[ItsDoot]

This might be another issue, but should we also add support for PreFixedUpdate/PostFixedUpdate? This same mechanism could be used for that too.

Happy to look into it in a separate PR. 👍