LibWeb: Implement window.queueMicrotask(callback)

This API allows authors to schedule a serialized JS callback that will
get invoked at the next spec-allowed opportunity.

Userland/Libraries/LibWeb/HTML/EventLoop/Task.h

@@ -24,6 +24,7 @@ public:
         HistoryTraversal,
         IdleTask,
         PostedMessage,
+        Microtask,
     };
 
     static NonnullOwnPtr<Task> create(Source source, DOM::Document* document, Function<void()> steps)