feat: c8 error if useless ignore

This is a feature request.

Eslint has an option to error when there are useless ignores. It would be great to have something similar for c8 where it errors when there is a useless coverage ignore (statement or file).

I realize that with random test suites sometimes code paths might or might not be hit, so this has to be an option. In our case it would be a very handy feature.

I've noticed that coverage is recorded for ignored files, so there might be an easy way to implement this as a plugin (if c8 has something like that). I'm not very familiar with the source code of this project, but maybe someone has some thoughts here.

Thank you very much! Really appreciate all the work done here and this project! L~