Use hooks + immutable components pattern for state transitions

## What problem does this solve or what need does it fill?

The current schedule-based approach to state transitions is messy.
It allows for system ordering ambiguities (#13947), and forces users to reason about a once-per-frame `StateTransition` schedule (which other crates may add more copies of).

State transitions hold important global invariants: we should use our new tools to make it easier (and more consistent) to reason about them.

## What solution would you like?

1. Remove the `StateTransition` schedule.
2. Make `State` immutable (blocked on #17485).
3. Remove `NextState`, and make users change states via commands.
4. Use hooks to call the `OnEnter`/`OnExit`/`OnTransition` schedules when the state changes.
5. Remove `.init_state` in favor of simply calling `.init_resource` on states. (Use a deprecation notice to make this easier).

## What alternative(s) have you considered?

#18562 discusses the possibility of adding another schedule before the initial state transition to allow for startup systems to run before states are entered.

I feel strongly that we shouldn't do this: it should be impossible to be in a state without its on-enter systems to be called (and similarly for on-exit). Instead, we should enforce this invariant as strongly as possible, and ensure that users have the tools needed to delay entering into states until any requisite setup is complete.

## Additional context

Note that this conflicts with the desire to run something *before* states are entered stated in #18562. However, users could simply initialize their starting states in a system during `Startup`, rather than calling `.init_state`.