bring back the usage class check and introduce a magic comment for non-standard usage classes

I believe that this check was genuinely helpful, and I want to bring it back. To deal with the deliberate employment of custom usage types, there is a new magic comment. I believe that this should satisfy the concerns that led to the check's removal, and I don't expect its return to be controversial.

Although I personally would put the magic comment in the _main.cfg, I put the code for recognizing the magic comment before the code for the check, so that the magic comment can also be put in the scenario where the usage type is first employed.