fix(core): raise null events on root object, but only if machine is not "done". Fixes #754

[davidkpiano]

This PR does 3 things:

• Detects when a machine is "done" internally (isDone) and prevents any events from being raised once it is done (according to SCXML spec)
• Raises a null event for a transient transition if the root machine has a transient transition (see Top-level null events are not fired if current state has no null event transitions #754)
  • Test added for this
• Adds state.done (enabled from first point) + tests + docs (Detecting Final State without starting #328)