scripting: Make QTimer constructible
If a QObject is exposed to js using QJSEngine::newQMetaObject(), a new instance of it can be made only with constructors exposed by Q_INVOKABLE
At the moment, QTimer's constructor is not Q_INVOKABLE, so code such as
const timer = new QTimer()
will not work.
BUG: 439630 FIXED-IN: 5.22.4