Add support for linting `qss` file

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

To address this, consider creating a pre-commit hook that wraps a tool like [`qsslint`](https://github.com/KDABLabs/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](https://doc.qt.io/qtforpython-6/PySide6/QtWidgets/index.html#list-of-classes)
- [`Qt for Python` QtGui](https://doc.qt.io/qtforpython-6/PySide6/QtGui/index.html#list-of-classes)

Start by reviewing the repository's existing pre-commit hooks and the qsslint project referenced in the issue. Check how qsslint can handle QSS without relying on Qt for Python or the private QtGui/private/qcssparser_p.h header; done means qss files are linted through the project's pre-commit workflow.

Written by the indexing model from the issue text.