[@xstate/fsm] Add support for targetless transitions

[davidkpiano]

This PR adds support for transitions with undefined targets, which will not exit/re-enter the state.

// ...
someState: {
  entry: [/* ... */],
  exit: [/* ... */],
  on: {
    SOME_EVENT: {
      // undefined target - will not exit/re-enter
      actions: [/* ... */]
    }
  }
}
// ...

[changeset-bot]

🦋 Changeset detected

Latest commit: 349b657

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

This PR includes changesets to release 1 package

Name	Type
@xstate/fsm	Minor

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[Andarist]

As I don't believe we are dealing with internal transitions here I would also adjust the PR title to avoid that term

[cblanc]

Thanks! 🙏