Various improvements

[Aloso]

This PR removes some string allocations in the supports_color function.

I originally wanted to use Result::as_deref for this, but since I wasn't sure what the minimum supported Rust version is, and Result::as_deref is only available since 1.47, I just wrote a helper function instead.

Other changes:

• I removed min since it is guaranteed to be 0 everywhere where it is used
• I reordered some code paths to do less work by returning earlier if FORCE_COLOR or NO_COLOR is set
• I fixed the tests, which were flaky; cargo runs tests in parallel, which does not work properly if multiple tests read and write environment variables. The solution was to add a lock that ensures the tests run sequentially.

[zkat]

This is awesome, and a welcome change! Thanks a lot!

[zkat]

published as supports-color@1.3.1 :)

[Aloso]

Thank you!