[Feature Request] Add a way to cleanup the the persistent directory

[nakibrayane]

After removing persisted files and directories when they are not needed any more, one have to manually delete them from the persistent directory, I know this is interned behavior, You don't want to delete a user's files.

But after a wile of using impermanence, the persistent directory become filled with unwanted files, if not maintained regularly, and cleaning it up would take a lot of time.

What I want is a script that can be run and list the directories and files that are in the persistent directory, and not in impermanence config, so that I can check the directories and files if they are important, And if not I can delete them manually.

This could look like something like this:

$ nix run github:nix-community/impermanence#collect-garbage -- --flake /etc/nixos#hostname
Directories in /persistent that are not persisted using impermanence module:
/foo
/bar
/foo/a.yaml
/bar/b.toml