Added some built-in states for Functions to return to navigate within the states.

- returning `fsm:next` transitions to the 'next' state by invoking the `next()` method.
- returning `fsm:prev` transitions to the 'previous' state by invoking the `previous()` method.
- returning `fsm:first` transitions to the 'first' state by invoking `state(first())`.
- returning `fsm:last` transitions to the 'last' state by invoking `state(last())`.