allow a deeper integration with nushell

As a maintainer of nushell, I just want to say that we love the effort of uutils/coreutils. Good work y'all and please keep up this excellent work!

We love it so much that we really want to have a much tighter integration than just calling external executables. I myself, have started to experiment by using the uu_cp and uu_mv crates as internal nushell commands in this PR https://github.com/nushell/nushell/pull/9463.

There are some rough edges that we'd need to work together on and I've tried to document different strategies in my PR. Without going into a lot of detail I'll just list a few things.

• Nushell doesn't use clap so working with command line arguments and the uu Options structs is interesting. Right now, I've kind of hacked it together where nushell checks params, then uu checks params, which isn't ideal, but it's a start.
• I've experimented with calling a command's uumain directly and passing args to it (and that can work), or rolling our own type of uumain. The benefit with rolling-our-own is we get the expected nushell "pretty" error messages. Error handling in general will need help and I'm not quite sure what to do here, specifically in regards to passing back a Span so we know where the error is.
• Streaming. Some uu_ commands stream output as output is generated, this is great and we'd need a way to hook into this output in order to consume it as nushell data structures that generate the nushell tabular or record output. I'm not really sure what is appropriate here.

In general, as you might guess, some things in coreutils may need to be exposed publicly, like command Options, to enable others, like nushell, to utilize them, and other things may need a different level of surgery in nushell or in coreutils.

One thing is for sure, we'd love to partner up and find a way forward because, let's be honest, dealing with some of these file-io and other commands is just difficult, and it would just be great to have one source of truth that works cross-platform like the uutils/coreutils.

Start by reading the linked Nushell PR #9463 and its documented integration strategies, then compare how uu_cp, uu_mv, uumain, and the command Options structs are used. Focus first on the unresolved argument handling, error propagation, and streaming questions. Done requires an agreed integration path between Nushell and coreutils, but the issue does not define a narrower implementation target.