Change output directory of pyside6-qmltyperegistrar

When running `pyside6-project qmllint` the `pyside6-qmltyperegistrar` script creates QML type files in a directory dictated entirely by the class QML import name (`
QML_IMPORT_NAME`).
For projects where the QML files start in the project root this makes complete sense, but in cases where your QML files start in a subdirectory (e.g. `./qml`), it would be ideal if the location of these generated files could be set relative to that location.

Especially for Python + QML projects there's real value in separating Python and QML files, so while not a game-breaking issue by any means, it would be lovely to have the option.