Implement RectangularShadow

Make poc of RectangularShadow element into QtQuick.Effects module. The need for this kind of element are shadows & glows for rectangular items such as buttons, panels etc. This sort of shadow is much more performant than traditional blurring method.

This would be somewhat similar to https://doc.qt.io/qt-6/qml-qt5compat-graphicaleffects-rectangularglow.html but brought into Qt6 and implemented as a C++ class similar to MultiEffect. Also the API could be made to match more closely the CSS box-shadow to make it more usable.

Considerations:

• Should there also be RectangularInnerShadow or "bool inner" property?
• Should there also be RectangularShadowGroup that would group multiple RectangularShadow elements and create a single shader effect from these?