(recs) make rxjs systems granularly unsubscribable

[smallbraingames]

Currently, defineRxSystem in [recs](https://github.com/latticexyz/mud/blob/main/packages/recs/src/System.ts) does not return the rxjs Subscription object, but only registers the unsubscribe to world.registerDisposer()

This prevents the ability to granularly unsubscribe from systems, which is useful in many situations, like building a game with different game modes, for example.

Easy possible fix: Return Subscription object. Current workaround:

function defineRxSystemUnsubscribable<T>(
  world: World,
  observable$: Observable<T>,
  system: (event: T) => void
): Subscription {
  const subscription = observable$.subscribe(system);
  return subscription;
}

export function defineComponentSystemUnsubscribable<S extends Schema>(
  world: World,
  component: Component<S>,
  system: (update: ComponentUpdate<S>) => void,
  options: { runOnInit?: boolean } = { runOnInit: true }
): Subscription {
  const initial$ = options?.runOnInit
    ? from(getComponentEntities(component)).pipe(toUpdateStream(component))
    : EMPTY;
  return defineRxSystemUnsubscribable(
    world,
    concat(initial$, component.update$),
    system
  );
}