qt_generate_foreign_qml_types(): Add flag to specify ownership

In the final example at https://doc.qt.io/qt-6/qqmlengine.html#QML_SINGLETON, the QML_FOREIGN() struct sets QJSEngine::CppOwnership in the create() function.

qt_generate_foreign_qml_types() should have a flag to set QJSEngine::CppOwnership in the generated code.