Consider to add "webbundle" to Request destination

[hayatoito]

Consider to add a new type request destination, webbundle.

WebBundle Explainer is here.

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.