check that all directories are imported in `Cslib.lean`

[chenson2018]

While I think this action checks for this and leaves a comment, an actual standalone test would be nice.

[jessealama]

You can run the check locally with:

lake exe mk_all --lib Cslib --check

To make this more visible/discoverable, we could:

1. Add a shell script in scripts/check-imports.sh
2. Add a lake script in lakefile.toml (runnable via lake script run check-imports)
3. Note somewhere that you can run GitHub Actions locally via act, like so:

   act -W .github/workflows/lean_action_ci.yml

I'm not sure which approach we should take. I'm inclined to tweak the lakefile. What do you think?

[chenson2018]

(that last PR mention was a typo, please ignore)