Incorporate changes for priority-hints

- Renames the existing implementor-internal "priority" attribute on Request to "internal priority".
- Adds a new "priority" interface to RequestInit and Request for specifying an explicit priority hint.

These changes when combined with similar changes to HTML in whatwg/html#8470 obsolete the existing Priority Hints specification https://wicg.github.io/priority-hints/

This fixes #1319

fetch.bs

@@ -4254,7 +4254,7 @@ the request.
 
   <p class=note>The <a>implementation-defined</a> object could encompass stream weight and
   dependency for HTTP/2, priorities used in <cite>Extensible Prioritization Scheme for HTTP</cite>
-  for transports where it applies (including HTTP/3), and equivalent information used to prioritize 
+  for transports where it applies (including HTTP/3), and equivalent information used to prioritize
   dispatch and processing of HTTP/1 fetches. [[!RFC9218]]
 
  <li>