As a QML developer, I want to write my own qmllint plugins to do domain specific checks

Since Qt 6.4, qmllint features a plugin system, and has e.g. a plugin to check for common mistakes in QtQuick. While this is already helpful, many users create their own components (e.g. KDE's Kirigami), often with their own best practice guides.

It is unrealistic to integrate checks for those custom features into the core qmllint application; however, by stabilizing the existing plugin system and making it public, we can allow those users to create their own checks.

This will then subsequently help any users of those custom frameworks to avoid making mistakes, and consequently lead to a better developer experience.

To enable users to create their own plugins, we need to

1. avoid exposing internal types in the public API (instead: expose PIMPLed wrappers),
2. document the API and
3. provide one reference example on how to create such a plugin.