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/Forward.h

@@ -346,6 +346,7 @@ class MessageEventWrapper;
 class MessagePortWrapper;
 class MouseEventWrapper;
 class NodeWrapper;
+class PageTransitionEventWrapper;
 class PerformanceTimingWrapper;
 class PerformanceWrapper;
 class ProcessingInstructionWrapper;