LibWeb: Add the PageTransitionEvent interface and fire "pageshow" events

We now fire "pageshow" events at the appropriate time during document
loading (done by the parser.)

Note that there are no corresponding "pagehide" events yet.

Userland/Libraries/LibWeb/DOM/Window.h

@@ -83,6 +83,8 @@ public:
     float scroll_x() const;
     float scroll_y() const;
 
+    void fire_a_page_transition_event(FlyString event_name, bool persisted);
+
 private:
     explicit Window(Document&);