feat: Coroutine instances

[eonarheim]

===:clipboard: PR Checklist :clipboard:===

• 📌 issue exists in github for these changes
• 🔬 existing tests still pass
• 🙈 code conforms to the style guide
• 📐 new tests written and passing / old tests updated with new scenario(s)
• 📄 changelog entry added (or not needed)

==================

This PR gives users greater flexibility with Excalibur Coroutines!

• Optionally ask coroutines not to start immediately

  ex.coroutine(function* () { .. }, { autostart: false });

• New CoroutineInstance is returned that is also awaitable

  export interface CoroutineOptions {
    timing?: ScheduledCallbackTiming;
    autostart?: boolean;
  }
   
  export interface CoroutineInstance extends PromiseLike<void> {
    isRunning(): boolean;
    isComplete(): boolean;
    done: Promise<void>;
    generator: Generator<CoroutineInstance | number | Promise<any> | undefined, void, number>;
    start: () => CoroutineInstance;
    cancel: () => void;
    then: Thenable;
    [Symbol.iterator]: () => Generator<CoroutineInstance | number | Promise<any> | undefined, void, number>;
  }

  const co = ex.coroutine(function* () { 
    yield 100;
  });
  
  await co; // wait for coroutine to finish

• start()/cancel()coroutines

  const co = ex.coroutine(function* () { 
    yield 100;
  });
  
  co.start();
  co.cancel();
  
  await co;

• nested coroutines! (Nested coroutines do not start running, they are run with their parent)

  const result = ex.coroutine(function* () {
      yield 100;
      yield* ex.coroutine(function* () {
        const elapsed = yield 99;
      });
      yield 100;
  });

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