Bug 1839316: part 3) Add `RequestPriority` to <Request.webidl>. r=kershaw,webidl,saschanaz

It's required for `ScriptFetchOptions`, see
<https://searchfox.org/mozilla-central/rev/3424c000a7ff304b2d1efb8561a924232f7f12fc/js/loader/ScriptLoadRequest.h#58-60,85>.

In a following part, `RequestPriority` will be added to `RequestInit`,
hence the former is defined in the correct file.

Differential Revision: https://phabricator.services.mozilla.com/D182006

dom/webidl/Request.webidl

@@ -46,6 +46,7 @@ interface Request {
 };
 Request includes Body;
 
+// <https://fetch.spec.whatwg.org/#requestinit>.
 dictionary RequestInit {
   ByteString method;
   HeadersInit headers;
@@ -78,3 +79,4 @@ enum RequestMode { "same-origin", "no-cors", "cors", "navigate" };
 enum RequestCredentials { "omit", "same-origin", "include" };
 enum RequestCache { "default", "no-store", "reload", "no-cache", "force-cache", "only-if-cached" };
 enum RequestRedirect { "follow", "error", "manual" };
+enum RequestPriority { "high" , "low" , "auto" };