[whatwg/fetch] consider exposing and allowing script to set Request priority (#436)

Currently the browser makes a number of decisions about what network requests are high priority vs low priority.  We could expose this as a `Request.priority` attribute.

In addition, we could allow script to set this so that authors can hint that some requests are low priority.  For example, think of a `requestIdleCallback()` that is intended to avoid interfering with normal page operation, but it needs a network request.  It could use a "low" priority.

Another use case would be pre-caching resources at a low priority in a ServiceWorker's install event handler.  This would reduce any impact to the currently displayed page.

I don't think we would want to guarantee honoring the value, though.  It would be a hint and best effort by the browser.

Webidl sketch:

```
interface Request
 {
  readonly attribute RequestPriority priority;
};
dictionary RequestInit
 {
  RequestPriority priority;
};
enum RequestPriority { "low", "normal", "high" };
```

Service workers would also be able to inspect priority:

```
addEventListener('fetch', evt => {
  if (evt.request.priority === 'high') {
    // high priority request
  }
});
```

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