add subscribe method to interpreter

src/index.ts

@@ -46,6 +46,7 @@ export interface Interpreter_T<C> {
   isTransitioning: boolean;
   isPaused: boolean;
   start: ()=>Interpreter_T<C>;
+  subscribe: (callback:SubscriptionCallbackFunction_T<C>)=>Interpreter_T<C>;
 }
 
 /**
@@ -62,6 +63,7 @@ export function Interpreter<C>(machine:Machine_T<C>, initialContext:any, initial
   //@ts-expect-error
   const interpreter : Interpreter_T<C> = {machine, state: initialStateName, context:initialContext, eventQueue:[], isTransitioning:false, subscriptions: {}, isPaused: true};
   interpreter.start = ()=>{ start(interpreter); return interpreter; }
+  interpreter.subscribe = (callback)=>{ subscribe(interpreter,callback); return interpreter; }
   send(interpreter, ['entry', null] );
   return interpreter;
 }