Run fixed time-step in an exclusive system

[joseph-gio]

Objective

• Allow combining fixed time-steps with run criteria.
• Lets us yeet looping run criteria in the future.

Solution

• SubSchedule -- a schedule that does not necessarily run along with the main schedule.
  • Gets stored in a public resource, and then extracted and run in an exclusive system.
  • Useful not just for fixed time-steps -- they could be used to invoke a collection of systems as a sort of callback, at any time. Example: OnEnter or OnExit for states.
  • Analogous to the system sets on #4391 -- only SubSchedule is worse since it's split into stages.
    • Eventually this API should be fully replaced in stageless. The primary motivation of this API is to ease migration.

---

Changelog

todo

Migration Guide

Before:

App::new()
    .add_system_set(
        SystemSet::new()
            .with_run_criteria(FixedTimestep::step(1.0 / 60.0))
            .with_system(my_system),
    );

After:

App::new()
    .add_fixed_schedule(
        FixedTimestep::step(1.0 / 60.0),
        SystemStage::single_threaded().with_system(my_system),
    );

More complex schedules:

#[derive(ScheduleLabel)]
struct FixedUpdate;

#[derive(StageLabel)]
struct FixedStage {
    A,
    B,
}

App::new()
    .add_fixed_schedule_to_stage(
        CoreStage::PreUpdate,
        FixedTimestep::steps_per_second(60.0).with_label(FixedUpdate),
        Schedule::default()
            .with_stage(FixedStage::A, SystemStage::parallel())
            .with_stage(FixedStage::B, SystemStage::parallel()),
    )
    .sub_schedule(FixedUpdate, |s: &mut Schedule| {
        s.add_system_to_stage(FixedStage::A, foo);
        s.add_system_to_stage(FixedStage::B, bar);
    });

Notes

This PR adds thiserror as a direct dependency for bevy_app, but it was already in the dependency tree.