Add a report that shows ignores without matches

[temp]

Is your feature request related to a problem?

The problem is that in long running projects there will be a lot of ignores that might be outdated, because rulesets changed.
These can only be resolved very cumbersome, by removing single ignores and running phpcs afterwards.

Describe the solution you'd like

In #808 @jrfnl suggested a specialized report that only highlights ignores without matches. This would be a huge help.
It would be nice to have this on-the-fly like phpstan does, but with the current codebase this is not possible.

• I have read the Contribution Guidelines and this is not a support question.
• I intend to create a pull request to implement this feature.