Replace atty dependency with std::io::IsTerminal.

[kpreid]

What

The atty library has a GitHub security advisory noting that it is unmaintained and can be unsound under some conditions. Its functionality can be replaced with std::io::IsTerminal; this PR does that.

I also added a helper function which replaces the two identical uses; this gives a single place to change the policy if desired, rather than perhaps accidentally editing only one site.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[teh-cmc]

Thanks

Checks:

• cargo r -p rerun-loader-rust-file -- run_wasm/src/main.rs does nothing
• cargo r -p rerun-loader-rust-file -- run_wasm/src/main.rs | bat prints a binary blob