[w3c/ServiceWorker] Allow using async event handlers instead of waitUntil() and respondWith() (#1235)

Calling `waitUntil()` on `event` doesn't sound right to me. Since the event has already fired, how is it that it has to "wait"? In addition, the code that one has to write is bulkier using `waitUntil()` and `respondWith()`. Why have `waitUntil()` code:
```javascript
self.addEventListener('install', function(event) {
  event.waitUntil(cache.addAll(urls));
});
```
when code could look like:
```javascript
self.addEventListener('install', async function(event) {
  await cache.addAll(urls);
});
```
and `respondWith()` code:
```javascript
self.addEventListener('fetch', function(event) {
  event.respondWith(
    caches.match(event.request).then(function(response) {
      return response || fetch(event.request);
    })
  );
});
```
could be written as:
```javascript
self.addEventListener('fetch', async function(event) {
  let response = await caches.match(event.request);
  return response || fetch(event.request);
});
```
Being able to use async functions to their full potential as event handlers would lead to code that is a lot easier to read and write.

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https://github.com/w3c/ServiceWorker/issues/1235