LibWeb: Add internals.wheel() to simulate mouse wheel events

This function allows us to write tests for scrolling scenarios where
the events are processed by the EventHandler.

Userland/Libraries/LibWeb/Internals/Internals.idl

@@ -11,6 +11,7 @@
     undefined commitText();
 
     undefined click(double x, double y);
+    undefined wheel(double x, double y, double deltaX, double deltaY);
 
     boolean dispatchUserActivatedEvent(EventTarget target, Event event);