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/Forward.h

@@ -195,6 +195,7 @@ class MessageChannel;
 class MessageEvent;
 class MessagePort;
 class PageTransitionEvent;
+class PromiseRejectionEvent;
 class SubmitEvent;
 class WebSocket;
 }
@@ -400,6 +401,7 @@ class PerformanceTimingWrapper;
 class PerformanceWrapper;
 class ProcessingInstructionWrapper;
 class ProgressEventWrapper;
+class PromiseRejectionEventWrapper;
 class ResizeObserverWrapper;
 class ScreenWrapper;
 class ScriptExecutionContext;