Part of: nushell/nushell#10446

nushell wants access to our mv internals. Which means exposing the exec function and Behavior struct. Feel free to rename those to something more sensible for a public API (e.g. move & Options, to match the naming scheme from cp). There should be documentation for all the public items in the API as well.

This is an excellent first issue: feel free to claim this!

We did the same thing for cp already. You could use that PR as a guide: #5152