Make StateTransitionSteps public (#16612)

# Objective

- Fixes #16594 

## Solution

- `StateTransitionSteps` was made public

## Testing

- _Did you test these changes? If so, how?_
 I am now able to import it and use it.

---

## Showcase

- Users can now write their own state scoped resources

```rust
    fn init_state_scoped_resource<R: Resource + FromWorld>(
        &mut self,
        state: impl States,
    ) -> &mut Self {
        use bevy::state::state::StateTransitionSteps; // this PR in action

        self.add_systems(
            StateTransition,
            (
                clear_state_scoped_resource_impl::<_, R>(state.clone())
                    .in_set(StateTransitionSteps::ExitSchedules), // and here
                init_state_scoped_resource_impl::<_, R>(state)
                    .in_set(StateTransitionSteps::EnterSchedules), // here too
            ),
        );

        self
    }
```

crates/bevy_state/src/state/transitions.rs

@@ -68,7 +68,7 @@ pub struct StateTransitionEvent<S: States> {
 ///
 /// These system sets are run sequentially, in the order of the enum variants.
 #[derive(SystemSet, Clone, Debug, PartialEq, Eq, Hash)]
-pub(crate) enum StateTransitionSteps {
+pub enum StateTransitionSteps {
     /// States apply their transitions from [`NextState`](super::NextState)
     /// and compute functions based on their parent states.
     DependentTransitions,