[core] Add ability to spawn actors in lazy initial context

[davidkpiano]

Actors can now be spawned directly in the initial machine.context using lazy initialization, avoiding the need for intermediate states and unsafe typings for immediately spawned actors:

const machine = createMachine<{ ref: ActorRef<SomeEvent> }>({
  context: () => ({
    ref: spawn(anotherMachine, 'some-id') // spawn immediately!
  })
  // ...
});

[changeset-bot]

🦋 Changeset detected

Latest commit: 1a978a0

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
xstate	Minor

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