Create a QT_IMPORT_QML_PLUGIN C++ macro for static qml plugin importing

Context
https://git.qt.io/alcroito/qt6-cmake-api-review/-/commit/14b4999bc857f5402a3577fe334ebfb3602ad0c2#f3ff9c1b48af14adf093e2af0c5cf2849829bb6d_176_232

For the use case of allowing multiple qml modules attached to an executable, we agreed that it should be achieved by creating multiple static qml modules and plugins and the plugins should be linked to the executable.

To ensure symbols are not discarded, Q_IMPORT_PLUGIN calls need to be added to the executable's source code, manually by the project developer.

To facilitate an upgrade path where that might be done automatically by the build system, developers should instead use QT_IMPORT_QML_PLUGIN instead of Q_IMPORT_PLUGIN.

This allows us to make QT_IMPORT_QML_PLUGIN a no-op in future Qt versions when the initialization will be done automatically by the build system, without causing duplicate symbol errors.

QT_IMPORT_QML_PLUGIN's implementation for Qt 6.2 should merely call QT_IMPORT_PLUGIN.