Add the ability to step through groups of parallel systems

One wishlist item from me is to support a way to step all parallel systems together, so we provide ways to try to replicate the parallel nature of the schedule. For game devs coming from an engine like Unity or Unreal, where everything is inherently serialized, may find it hard to bridge the gap to Bevy, so providing a debugging option that lets you step through groups of parallel systems may help.

_Originally posted by @james7132 in https://github.com/bevyengine/bevy/pull/8453#pullrequestreview-1860828081_