[whatwg/fetch] Alt-Used header conflict handling (#1272)

The other day I learned about https://datatracker.ietf.org/doc/html/rfc7838#section-5. It lacks Fetch integration. What happens if a web developer sets that header and later the browser wants to set it for some reason? Does the browser override, fail?

cc @whatwg/http @martinthomson 

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