qmltc: support C++ namespaces

(Splitted from QTBUG-91956)

Test and support C++ namespaces in qmltc. Qml Types defined in a C++ namespace should be visible and usable from qmltc.

• C++ namespaces (a C++ type exposed to QML could be declared inside a namespace; that namespace should be visible and usable by the compiler)
• at least requires testing; mostly relies on qmlcompiler lib?