Add ability for a folder to be marked as ignored by kondo

[juh9870]

Would be nice to have an ability to create some sort of marker file in a directory (eg. .kondokeep), that would signal to kondo to not clean node_modules/target/etc from the folder.

My use case:
I want to regularly use kondo to clean up projects that weren't used in a week or two, but I have an important Node.js project, in which I need to preserve node_modules, otherwise some of my workflow breaks. What's worse, is that I also use syncthing to sync this project to multiple workstations to different paths, so I don't even have a reliable absolute path to ignore for kondo. I would love to just put some sort of .kondokeep into the project folder and be done with it.

GUI note:
Since this file can be inserted maliciously or by accident (aka, committed to git and then cloned), it would probably require different treatment from the -I flag. I suggest that directories ignored this way still show up in the list of folders in the GUI/CLI, but you just can't delete them. This would allow a user to see if any of their projects is using this .kondokeep, and clean it up manually if needed.

An extra idea is to also show the content of the .kondokeep in the CLI/GUI, so project authors can put some explanation into that file as to why this project should not be cleaned with kondo.