LibWeb: Add @@toStringTag to platform object prototypes

This was forgotten to be added in the LibWeb GC conversion.
This caused some brand checks to fail in skribbl.io's JavaScript and
thus caused unexpected exceptions.

Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp

@@ -2574,6 +2574,8 @@ void @prototype_class@::initialize(JS::Realm& realm)
     }
 
     generator.append(R"~~~(
+    define_direct_property(*vm.well_known_symbol_to_string_tag(), js_string(vm, "@name@"), JS::Attribute::Configurable);
+
     Object::initialize(realm);
 }
 )~~~");