This adds support for clearing events when **entering** a state (instead
of just when exiting) and updates the names to match
`DespawnOnExitState`.
Before:
```rust
app.add_state_scoped_event::<MyGameEvent>(GameState::Play);
```
After:
```rust
app
.add_event::<MyGameEvent>()
.clear_events_on_exit_state::<MyGameEvent>(GameState::Play);
```
