Linter - have a mechanism to detect "orphan" rules

With the new linter system, you have to add rules to a ruleset and enable them for them to have an effect. One distinct possibility here is that you add a rule and forget to add it to a ruleset, so it never has any effect.

We should have some kind of mechanism that will alert us when a rule is not part of any ruleset. This could be an on-demand script, or possibly a CI check (which would prevent you from checking in a rule without adding it to SOME ruleset).