LibJS: Implement SetDefaultGlobalBindings as a standalone function

Instead of hardcoding all the property definitions in GlobalObject's
initialize() function, make it the standalone AO it is supposed to be
that can then be used by other global objects that don't inherit from
JS::GlobalObject.

Userland/Libraries/LibJS/Runtime/GlobalObject.h

@@ -40,6 +40,8 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(unescape);
 };
 
+Object& set_default_global_bindings(Realm&);
+
 template<>
 inline bool Object::fast_is<GlobalObject>() const { return is_global_object(); }