Add a `--check` subcommand

First of all, thank you for creating this useful tool!

I've been experimenting with wit-deps for a while. At the moment, using it as a tightly integrated dependency manager for my wit deps from within my Rust deps doesn't quite work yet. Therefore, I'm trying to switch to just using wit-deps in CI for now to assert that my deps.lock file is accurate and my deps folder is up-to-date. At the moment, it seems that checking the deps folder requires running `wit-deps update`, which fully recreates the folder. In my usecase, this wont't work, unfortunately.

Would it be possible to add a `wit-deps check` subcommand? It should work almost identically to `wit-deps update`, but instead of overriding folders and files, it would check that their contents are the same. Crucially, it should follow symlinks.

Thanks for your help and feedback :)