LibWeb: Implement PromiseRejectionEvent

This paves the way for the rejectionhandled and unhandledrejection
events.

It's also used by core-js (in browsers, at least) to check whether
Promise needs to be polyfilled, so adding it should allow more websites
to leverage LibJS's native Promise implementation :^)

Userland/Libraries/LibWeb/HTML/PromiseRejectionEvent.idl

@@ -0,0 +1,14 @@
+#import <DOM/Event.idl>
+
+[Exposed=(Window,Worker)]
+interface PromiseRejectionEvent : Event {
+    constructor(DOMString type, PromiseRejectionEventInit eventInitDict);
+
+    readonly attribute Promise<any> promise;
+    readonly attribute any reason;
+};
+
+dictionary PromiseRejectionEventInit : EventInit {
+    required Promise<any> promise;
+    any reason;
+};