Hard-code localhost to loopback addresses

Incorporate the recommendation of https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-let-localhost-be-localhost to hard-code the resolution of *.localhost domains to ::1 and 127.0.0.1. This allows user agents to ensure that localhost contexts meet the secure context requirements laid out in https://w3c.github.io/webappsec-secure-contexts/#localhost.

Author: mikewest

fetch.bs

@@ -2207,11 +2207,21 @@ unset or <a for=request>keepalive</a> is false, <a lt=terminated for=fetch>termi
 <h3 id=resolving-domains>Resolving domains</h3>
 
 <p tracking-vector>To <dfn>resolve a domain</dfn>, given a <a for=/>network partition key</a>
-<var>key</var> and a <a for=/>domain</a> <var>domain</var>, perform an <a>implementation-defined</a>
-operation to turn <var>domain</var> into a <a for=/>set</a> of one or more
-<a for=/>IP addresses</a>. If this operation succeeds, return the <a for=/>set</a> of
-<a for=/>IP addresses</a>. If it fails, return failure. The results of this operation may be cached.
-If they are cached, <var>key</var> should be used as part of the cache key.
+<var>key</var> and a <a for=/>domain</a> <var>domain</var>:
+
+<ol>
+ <li><p>If <var>domain</var> is a <a for=/>host</a> whose <a for=host>public suffix</a> is
+ "<code>localhost</code>", then return « <code>::1</code>, <code>127.0.0.1</code> ».
+
+ <li><p>Perform an <a>implementation-defined</a> operation to turn <var>domain</var> into a
+ <a for=/>set</a> of one or more <a for=/>IP addresses</a>. If this operation succeeds, return the
+ <a for=/>set</a> of <a for=/>IP addresses</a>.
+
+ <li><p>Return failure.
+</ol>
+
+<p>The results of <a>resolve a domain</a> may be cached. If they are cached, <var>key</var> should
+be used as part of the cache key.
 
 <div class=note>
  <p>Typically this operation would involve DNS and as such caching can happen on DNS servers without