qtshadertools module RTA

Follow-up to QTQAINFRA-3440, as recommended by Jani H.

qt/qtshadertools is a new module in dev (Qt 6.0).

When building, it produces a regular Qt module (shadertools, with corresponding shared library), and a command-line tool called qsb. We expect the latter to ship with Qt like other similar tools (such as, qmlscene, for example).