LibJS: Move builtin prototypes to the global object

This moves us towards being able to run JavaScript in different global
objects without allocating a separate GC heap.

Libraries/LibWeb/Bindings/WindowObject.cpp

@@ -43,6 +43,12 @@ namespace Bindings {
 WindowObject::WindowObject(Window& impl)
     : m_impl(impl)
 {
+}
+
+void WindowObject::initialize()
+{
+    GlobalObject::initialize();
+
     put("window", this);
     put_native_property("document", document_getter, document_setter);
     put_native_function("alert", alert);