Add support for linting `qss` file

[jcfr]

Currently, mirror-prettier does not support linting qss files.

To address this, consider creating a pre-commit hook that wraps a tool like qsslint.

Warning

Because the private class QCss (provided by QtGui/private/qcssparser_p.h) is required, relying on Qt for Python is not feasible.

For reference:

• Qt for Python QtWidgets
• Qt for Python QtGui