Add App.app/Contents/PlugIns/qml as a default qml import path for macOS apps

This would relieve the project developer from changing their C++ code to add a import path manually, as well as introduce a common way of doing it for all macOS apps.
Potentially this simplifies default bundling / deployment qml logic.

There is currently one qtdeclarative example that does that manually.
https://github.com/qt/qtdeclarative/blob/dev/examples/qml/tutorials/extending-qml/chapter6-plugins/main.cpp#L60

The discussion popped up before at
https://codereview.qt-project.org/c/qt/qtdeclarative/+/366961/1/examples/qml/tutorials/extending-qml/chapter6-plugins/Charts/CMakeLists.txt#23
and
https://codereview.qt-project.org/c/qt/qtdeclarative/+/366961/5/examples/qml/tutorials/extending-qml/chapter6-plugins/Charts/CMakeLists.txt#22

A few other examples currently set 'PlugIns', not 'PlugIns/qml' as an import path manually.
https://github.com/qt/qtdeclarative/blob/dev/examples/quick/shared/shared.h#L58
https://github.com/qt/qtdeclarative/blob/dev/examples/quick/shared/QtBundleQmlModuleForMacOS.cmake#L15