Sec-Purpose is not set for prerenders

[domenic]

Somehow we seem to have closed #133 without actually writing any spec text for it? This is bad!

Suggested plan:

• Define a new prerender token in https://wicg.github.io/nav-speculation/prefetch.html#http-headerdef-sec-purpose
  • This is the Prefetch spec, not the Prerender spec. However, I think it is best to keep the definition of Sec-Purpose centralized.
  • Bonus: update that section to be a monkeypatch on top of https://fetch.spec.whatwg.org/#sec-purpose-header , which has been added to the Fetch spec since we originally wrote the Prefetch spec.
  • Bonus: update https://wicg.github.io/nav-speculation/prefetch.html#create-navigation-params-by-fetching to not mention a list of tokens, and thus be a bit more similar to the Fetch spec definition in https://fetch.spec.whatwg.org/#ref-for-http-sec-purpose%E2%91%A0
• Update https://wicg.github.io/nav-speculation/prerendering.html#navigate-fetch-patch to add the header
  • This section is modifying https://html.spec.whatwg.org/#create-navigation-params-by-fetching . I think we basically want to insert a new step after step 3 or 4 of that algorithm.
  • The new step should use similar language to https://wicg.github.io/nav-speculation/prefetch.html#create-navigation-params-by-fetching steps 17.4.1-3 , although possibly modified per the above list vs. single-token issue.