Send Sec-Purpose: prefetch for prefetch requests

The purpose headers allows servers to discern between regular requests
and "resource hints" - requests that are only meant to populate
caches.

This replaces the existing `Purpose` and `x-moz: prefetch` headers.

Closes w3c/webappsec-fetch-metadata#84
Part of whatwg/html#8111

fetch.bs

@@ -5039,6 +5039,10 @@ steps. They return a <a for=/>response</a>.
    <li><p><a abstract-op lt="append the Fetch metadata headers for a request">Append the Fetch metadata headers for <var>httpRequest</var></a>.
    [[!FETCH-METADATA]]
 
+   <li><p>If <var>httpRequest</var>'s <a for=request>initiator</a> is "<code>prefetch</code>", then
+   <a for="header list">append</a> (`<code>Sec-Purpose</code>`, "<code>prefetch</code>") to
+   <var>httpRequest</var>'s <a for=request>header list</a>.
+
    <li><p>If <var>httpRequest</var>'s <a for=request>header list</a>
    <a for="header list">does not contain</a> `<code>User-Agent</code>`, then user agents should
    <a for="header list">append</a> (`<code>User-Agent</code>`,