Transition<Event, State>  constructor 
      const
      Transition<Event, State> ({ 
    
- required State currentState,
- required Event event,
- required State nextState,
A Transition is the change from one state to another.
Consists of the currentState, an event, and the nextState.
Implementation
const Transition({
  required State currentState,
  required this.event,
  required State nextState,
}) : super(currentState: currentState, nextState: nextState);