Add command to list contents of a sparse dat repository

[mitar]

Even before we have downloading of individual files (https://github.com/datproject/dat/issues/796), we can do something like:

dat clone dat://<key> --empty
dat pull --select <path/to/file>

But how can one know which files are there?

I suggest something like dat ls is added to list all files in the (sparse) dat repository.

[millette]

Although I haven't touched dat-shell in a few months, it is probably worth it to explore which features could get merged back into dat itself.

With dat-shell, you can:

dat-shell dat://key/path/somewhere # in your OS shell
ls # in dat-shell