Add Sec-Fetch-Dest header name constant

[invliD]

See fetch spec, which is the defining spec according to the relevant IANA entry.

[seanmonstar]

Thanks for the PR!

I'm not sure we can add every header name, the goal was to list the most common. Though, there is #573 to better define this policy.

[invliD]

I had read that thread, and I was under the impression that headers registered as "permanent" with IANA are fair game. Feel free to close this PR if you don't want this one in here.