LibWeb: Implement the JS host hooks for promises, job callbacks and more

This overrides the JS host hooks to follow the spec for queuing
promises, making/calling job callbacks, unhandled promise rejection
handling and FinalizationRegistry queuing.

This also allows us to drop the on_call_stack_emptied hook in
Document::interpreter().

Userland/Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp

@@ -284,7 +284,8 @@ void EventLoop::perform_a_microtask_checkpoint()
 
     // FIXME: 5. Cleanup Indexed Database transactions.
 
-    // FIXME: 6. Perform ClearKeptObjects().
+    // 6. Perform ClearKeptObjects().
+    vm().finish_execution_generation();
 
     // 7. Set the event loop's performing a microtask checkpoint to false.
     m_performing_a_microtask_checkpoint = false;

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

@@ -26,6 +26,7 @@ public:
         PostedMessage,
         Microtask,
         TimerTask,
+        JavaScriptEngine,
     };
 
     static NonnullOwnPtr<Task> create(Source source, DOM::Document* document, Function<void()> steps)