Use Web IDL's new-ish interface mixins concept (#1252)

WebIDL recently introduced dedicated syntax for mixins [1]. So, this
change replaces the existing `implements` with `includes` as per the new
concept.

[1] whatwg/webidl@45e8173

docs/index.bs

@@ -302,7 +302,7 @@ spec: webappsec-referrer-policy; urlPrefix: https://w3c.github.io/webappsec-refe
         // event
         attribute EventHandler onstatechange;
       };
-      ServiceWorker implements AbstractWorker;
+      ServiceWorker includes AbstractWorker;
 
       enum ServiceWorkerState {
         "installing",