cp: compile without clap (for nushell)

[tertsdiepraam]

Context: nushell/nushell#10097

All the utils include a clap-based argument parser. If we want to expose the internals of the utils, it should also be possible to only compile the internals without the argument parser and the clap dependency. This is a request from nushell, where they want to reuse our utils with their own argument parsing.

So, we should introduce a feature (e.g. called "cli", "parser" or "uumain", I'm not sure), which enables the uumain function and the clap parser. This should probably be enabled by default (nushell can then include it with default-features = false).

To compile entirely without clap, uucore also needs to be able to compile without clap. This issue is therefore also relevant: #5202.