Add support for single-state (combinatorial) machines

[davidkpiano]

This PR adds support for combinatorial machines:

• No initial property
• No state property
• Root-level transitions

const testMachine = createMachine({
  context: { value: 42 },
  on: {
    INC: {
      actions: assign({ value: (ctx) => ctx.value + 1 })
    }
  }
});

The finite state.value of the combinatorial machine is an empty object: {}

[changeset-bot]

🦋 Changeset detected

Latest commit: e37fffe

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

This PR includes changesets to release 3 packages

Name	Type
xstate	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[Andarist]

This almost feels like a bug fix to me 😅