Migrate from `atty` to `is-terminal`

[Joining7943]

cargo deny gives an advisory warning about atty and it doesn't look like this will be fixed anytime soon because the atty crate doesn't seem to be actively maintained anymore.

is-teminal in contrast is maintained, is a port of atty with this bug fixed and has some additional features. It looks like the better choice and although the api is different, migrating uutils should not be a too big task. It's changing a line with atty::is(atty::Stream::Stdout) to std::io::stdout().is_terminal() (or other streams).

[tertsdiepraam]

Also is-terminal is already a dependency of clap, so it's already part of our dependency graph.

[souzaguilhermea]

If someone isn't already working on it, I'm interested to work on this issue.

[Joining7943]

Sure, you can go ahead as far as I'm concerned.