feat: Coroutine instances

[eonarheim]

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

• [ ] :pushpin: issue exists in github for these changes
• [x] :microscope: existing tests still pass
• [x] :see_no_evil: code conforms to the style guide
• [x] :triangular_ruler: new tests written and passing / old tests updated with new scenario(s)
• [x] :page_facing_up: 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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