LibWeb: Add most of ServiceWorker Update algorithm

This misses the final, most important part of actually creating a
service worker object and sending the script over to it in a WebWorker
process.

Tests/LibWeb/Text/input/ServiceWorker/service-worker-register.html

@@ -5,6 +5,9 @@
         setTimeout(() => {
             // We need to do this spoofing later in the event loop so that we don't end up
             // telling the test runner the wrong URL in page_did_finish_loading.
+
+            // FIXME: We need to use an actual https:// domain here in order to
+            //        actually fetch() our js file.
             spoofCurrentURL("https://example.com/service-worker-register.html");
 
             let swPromise = navigator.serviceWorker.register("service-worker.js");