[whatwg/fetch] Request destination for SVG documents (#1012)

It's unclear what `destination` value should be used for SVG documents and different browsers seem to be doing different things, at least when it comes to CSP.

https://svg-discrimination.glitch.me/csp.html is loading fine in Chrome and Safari (as long as `img-src: 'self'` is there), but the SVG is blocked in Firefox.

At the same time, because SVG document resources are internally represented differently than images in Chromium, an `as` value of "image" for preloads is a [mismatch for SVG documents](https://bugs.chromium.org/p/chromium/issues/detail?id=1065069).

Looking at [Fetching the document](https://www.w3.org/TR/SVG2/linking.html#processingURL-fetch) over in SVG-land doesn't mention `destination` at all.

We should decide what the destination be, and then align spec and implementations on it.
My personal vote would be for a brand new destination (e.g. "svg" or "svg-document"), to make it easier to explain to developers which CSP or `as` values control these loads.

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