Apply label only if it's the only change to the document

[mofosyne]

In https://github.com/jart/cosmopolitan/issues/1219 we had an issue where a label named 'testing' was triggering for commits with testing changes but is considered more than just testing. This makes the label a bit too noisy.

This can also apply to documentation PRs as well. Perhaps we only want to apply documentation and testing label if its a PR dedicated to only these changes.

Some way to add a mode where we are making an 'extra' exclusive match would be appreciated... to adjust below case to be specific to PRs with only changes to this folder.

testing:
    - changed-files:
        - any-glob-to-any-file:
            - test/**

Are you willing to submit a PR? Not intending to make a PR for this proposal for now.