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