LibWeb: Make the Window object "inherit" from EventTarget :^)

Since Web::Bindings::WindowObject inherits from JS::GlobalObject, it
cannot also inherit from Web::Bindings::EventTargetWrapper.

However, that's not actually necessary. Instead, we simply set the
Window object's prototype to the EventTargetPrototype, and add a little
extra branch in the impl_from() function that turns the JS "this" value
into a DOM::EventTarget*.

With this, you can now call window.addEventListener()! Very cool :^)

Fixes #4758.

Userland/Libraries/LibWeb/Bindings/EventTargetWrapperFactory.h

@@ -31,6 +31,6 @@
 
 namespace Web::Bindings {
 
-EventTargetWrapper* wrap(JS::GlobalObject&, DOM::EventTarget&);
+JS::Object* wrap(JS::GlobalObject&, DOM::EventTarget&);
 
 }