checkrefs.pl, while useful, is annoying:
- it's written in perl, which is arcane at the best of times.
- it's also annoying to run (I don't have libxml::simple or whatever its dependencies are and installing them is a pain).
- Running externally, one has to recreate a bunch of game functionality.
We could also leverage our GUI to give some neater information, though I'm really doing nothing of that kind right now.
This adds a mod to run some simple validations. I used it to confirm D1775.