Split islands in async task

[Jondolf]

Island splitting can be expensive for very large islands. We should try to do it in an async task that runs concurrently with the solver to hide the cost, like Box2D.

[Jondolf]

We would also need to move BodyIslandNode stuff into a resource, and support contact edge lookup with body entities instead of collider entities. This is because we can't access ECS queries inside the async task.