Use Qt for MCU compiler frontend in QDS.

We should use the 'QML to C++ compiler front-end' to validate the QML code for Qt for MCU.

This would be part of a private closed source plugin. This would allow running a verification step for Qt for MCU projects that catches issues.

This would require making the compiler front-end part of a QDS plugin, but this should be a lot easier than actually having full end-to-end support including MinGW.