[w3c/ServiceWorker] Async functions and waitUntil (#997)

Given latest comments in https://github.com/w3c/ServiceWorker/issues/920 I got some thoughts that it would be good to pass `async function` directly into `fetch` event.  

My questions is: is `waitUntil` automatically called when `async function` is used as an listener?
I would expect this to work:

```js
self.addEventListener('fetch', async function(e) {
  const preload = await e.navigationPreload;

  if (preload) {
    e.respondWith(preload);
  }
});
```

with manual `waitUntil` it needs to be:


```js
self.addEventListener('fetch', async function(e) {
  let preload = e.navigationPreload;
  e.waitUntil(preload);
  preload = await preload;

  if (preload) {
    e.respondWith(preload);
  }
});
```

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