Enable skipping/running SystemSets during Stepping

[cBournhonesque]

Objective

• Fixes Introduce an app::run_system_set() function #12717
• My objective was to be able to run a specific SystemSet inside a test (for example, to only run physics, etc.). There is already a run_schedule() function but nothing more specific
• I made a first attempt in Add World::run_system_set to run only the systems in a SystemSet #12726, but the solution wasn't very elegant

Solution

• Instead, I think it's possible to extend the existing system stepping machinery to achieve what I want. It should not be a problem to require system-stepping since run_system_set() would mostly be used inside tests.
  As a bonus, the new solution provides more control, since it's possible to run all the systems in a set except for one, etc.

• Add functions in SystemStepping to be able to add SystemStepping instructions (AlwaysRun, NeverRun) for an entire SystemSet. In those cases, system-stepping will issue instructions for every system that is contained inside the Set.

• For ease of computation I had to add a variable sets_to_systems in the ScheduleGraph (which is gated behind the system-stepping feature), but it's requiring a lot of #[cfg(feature(bevy_debug_stepping))] statements elsewhere in the code. I wonder if there's a better solution?

Testing

• Added some unit tests to check that system-set stepping instructions work properly

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Changelog

Added

Extended SystemStepping to be able to Skip/Step-through all the systems of a SystemSet