chore: replace atty with is-terminal

[tottoto]

Replaces atty with is-terminal as the former is unmaintained.

[chriskrycho]

Thanks for opening this! A couple notes:

1. I don't see any actual explicit indication on the repo that is-terminal is unmaintained, only a recommendation that folks use the Rust built-in post-1.70. Do you have some further info?
2. We're currently evaluating our MSRV, and if we bump it to a more recent version it's very likely we will just do exactly what is-terminal recommends and use the thing Rust ships.

[chriskrycho]

Oh! I see, I read this backwards; you're wanting to move this from atty to is-terminal. I think the same basic question applies, and in general I would not be worried by something having gone without an update for a while… but seeing as atty has a known soundness issue, I'm strongly in support here. I will rebase this and get it merged. Thanks!