Feature request: on_enter and on_exit events should match prev state and next state filters respectively.

[ewmb7701]

I would like to match the prev state for on_enter events and the next state for on_exit events.

My goal is an on_enter and on_exit event which only triggers when I enter/ exit a group of states, but not when I transition between those states. That would look something like this:

  .on_enter::<OneOfState<(
      PlayerAirMoveDownStateComponent,
      PlayerAirMoveUpStateComponent,
  )>, Not<OneOfState<(
      PlayerAirMoveDownStateComponent,
      PlayerAirMoveUpStateComponent,
  )>>(|_ec| {
      bevy::log::info!("enter air");
  })

[ewmb7701]

My specific use case is to insert/ remove a component for a set of states. I've realized I can achieve my desired behavior without any API changes like this:

  .on_enter::<OneOfState<(
      PlayerAirMoveDownStateComponent,
      PlayerAirMoveUpStateComponent,
  )>>(|ec| {
      ec.insert(InAirStateSetComponent);
  })
  .on_enter::<NotState<
      OneOfState<(
          PlayerAirMoveDownStateComponent,
          PlayerAirMoveUpStateComponent,
      )>,
  >>(|ec| {
      ec.remove::<InAirStateSetComponent>();
  })

I'm still keeping this issue open with the pull request, because I still think it's a good API.

[Seldom-SE]

Done in #19