[w3c/ServiceWorker] Tee-ing Cache.put()? (#1367)

IIUC, a common pattern developers would write when lazily populating a `Cache` would be something like:
```js
event.respondWith(fetch(url).then(response => {
  var responseClone = response.clone();
  cache.put(url, response);
  return responseClone;  
}));
```
With a new method `teePut` (placeholder name, better names welcome!) a developer could write:
```js
event.respondWith(fetch(url).then(response => cache.teePut(url, response)));
```
where `teePut` returns a `Response` (or maybe `Promise<Response>`) that is semantically a clone of the `Response` given as argument.

In addition to being simpler to write, this might be more optimizable by the browser for two reasons:
* the constrained form of the tee, compared to a general `clone()`, may allow less internal copying between source and sink streams
* if the response clone flows into one of [several](https://v8.dev/blog/improved-code-caching) [content](https://blog.mozilla.org/javascript/2017/12/12/javascript-startup-bytecode-cache/) [sinks](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/compileStreaming) that want to cache a processed form of the resource, the explicit tee operation provides a more-direct route from the content sink back to the Cache entry.

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