Replace startup systems with a built-in state enum that handles resource cleanup

# Problem

1. Startup systems are largely special-cased, they rely on custom stage implementations and it's quite hard to teach the model.
2. It is challenging to ensure appropriate cleanup of resources on `AppExit`: we must listen for the event and carefully time the resources.

## Proposed solution

```rust
/// A built-in game state that controls app control flow
///
/// This is enabled by default, and user systems are added to the `Main` variant by default
enum AppState {
Startup,
Main,
Exit,
}
```

This allows us to use standard on-enter, on-exit and state transition methods to control things like asset loading, cleanup, pausing the game, autosaving on exit and so on.

## Extensions

We could add an explicit asset loading state as well. This may work well, but could be controversial.

## Context

Discussed with @IceSentry on [Discord](https://discord.com/channels/691052431525675048/1000497773474496544/1000500300127408168) after trying to help a user ensure data is cleaned up correctly.

This builds on but does not require the patterns espoused by the [Stageless RFC}(https://github.com/bevyengine/rfcs/pull/45).

Would address #1353. If we were to implement this, we would likely want to have a more robust event strategy by default, to ensure that gameplay events do not get lost during a pause.