[whatwg/fetch] Consider to add "webbundle" to Request destination (#1120)

Consider to add a new type request destination, `webbundle`.

WebBundle Explainer is [here](https://github.com/WICG/webpackage/blob/master/explainers/subresource-loading.md).

`webbundle` destination should be used if and only if fetching a bundle from a `<link>` element whose `rel` attribute is `webbundle`, such as
```
<link rel=webbundle href="https://example.com/foo.wbn" resources="...">
```

By having that, a site who serves a webbundle, such as `example.com` in the example,  can distinguish a request from a `<link rel=webbundle ...>` and other cases. They might want to serve `foo.wbn` only when a request's destination is `webbundle`.


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