Add support for creating full Quick Rectangle nodes from C++

QQuickWindow contains createRectangleNode() method https://doc.qt.io/qt-6/qquickwindow.html#createRectangleNode

However, this creates QSGRectangleNode which only supports color property. What would also be useful is a method which creates QSGDefaultInternalRectangleNode so that user gets full QML Rectangle features with radius, border, gradient etc. Alternatively these features could be added into QSGRectangleNode, but then there wouldn't be a "simple rectangle" option anymore.

This could mean:

• Renaming QSGDefaultInternalRectangleNode to e.g. QSGDefaultRectangleNode (as current createRectangleNode can't be renamed to e.g. createSimpleRectangleNode) and making it public.
• Adding new QQuickWindow::createDefaultRectangleNode() method.