feat: add excessive-visibility lint

[aryansk]

Fixes #1207\n\n## Summary\n- add an experimental excessive-visibility design lint\n- flag public named methods that have no matching unit-test reference\n- include the requested warning motive and positive/negative YAML fixtures\n\n## Validation\n- xmllint --noout on the new XSL\n- Java/Maven validation was not available in the local environment; the repository CI can run the native lint fixture suite

[volodya-lombrozo]

Thanks for the contribution! I have to close this one, though — the commit isn't signed, and rultor requires signed commits before it can merge anything here. Could you sign your commits and reopen? Docs on how: https://docs.github.com/en/authentication/managing-commit-signature-verification/signing-commits

[aryansk]

Thank you for explaining the requirement. I will not reopen this without properly verified signed commits, so I am leaving the closed PR unchanged.