Replace atty with std::io::IsTerminal

[cpolzer]

Hey,

playing around with your nih-plug framework and stumbled over this...

Replace atty with std::io::IsTerminal

atty has a known unsoundness issue (GHSA-g98v-hv3f-hcfr) where it reads from uninitialized memory on Unix platforms.

Since Rust 1.70, std::io::IsTerminal is available in the standard library and covers the same use case. This replaces
the single atty::is(atty::Stream::Stderr) call with std::io::stderr().is_terminal(), removing the atty dependency
entirely.

Changes:

• Remove atty from Cargo.toml
• Replace atty::is(atty::Stream::Stderr) with std::io::IsTerminal::is_terminal(&std::io::stderr()) in src/target.rs

No behaviour changes.