feat: actionable error message for BREAKING CHANGE subject

⧗   input: BREAKING CHANGE: use recommended `heroku/builder:22`
https://github.com/heroku/cnb-builder-images#available-images
✖   move BREAKING CHANGE to footer [change-in-subject]
✖   add type [type-empty]
✖   add subject [subject-empty]
✖   found 2 problems, 0 warnings

⧗   input: BREAKING CHANGE: use recommended `heroku/builder:22`
https://github.com/heroku/cnb-builder-images#available-images
✖   subject may not be empty [subject-empty]
✖   type may not be empty [type-empty]
✖   found 2 problems, 0 warnings