QML tooling: Indicate which config settings were used

We have some elaborate way to find config files for QML tooling like qmlformat, qmllint, etc.

There should be a way to query the tool which config file is actually use; it was mentioned in private conversion that `–help` might not be suitable for it, as it might depend on the input.

One option would be a --dry-run option, which would print what the command is supposed to do, and which configuration file it will pick up.