[core] Ensure "stop" transitions are called in machine actors

[davidkpiano]

This PR ensures that "stop" transitions are called in machines:

const machine = createMachine({
  on: {
    'xstate.stop': {
      actions: () => {
        // do cleanup, etc...
        console.log('Machine actor was just stopped');
      }
    }
  }
});

const actor = createActor(machine).start();

actor.stop();
// Logs "Machine actor was just stopped"

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🦋 Changeset detected

Latest commit: 21f09e24f36a43cecdcb8223c47450077325b5e2

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

This PR includes changesets to release 1 package

| Name | Type | | ------ | ----- | | xstate | Patch |

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