LibWeb: Implement most of Service Worker registration

This approach will need some rework to be properly handled at the user
agent level instead of per renderer process, but it's a start.

Meta/gn/secondary/Userland/Libraries/LibWeb/ServiceWorker/BUILD.gn

@@ -1,5 +1,9 @@
 source_set("ServiceWorker") {
   configs += [ "//Userland/Libraries/LibWeb:configs" ]
   deps = [ "//Userland/Libraries/LibWeb:all_generated" ]
-  sources = [ "Job.cpp" ]
+  sources = [
+    "Job.cpp",
+    "Registration.cpp",
+    "ServiceWorker.cpp",
+  ]
 }