Better warnings/errors for allow-comments

[ZedThree]

Currently we just abort if an allow comment is not a valid rule, but we should instead raise a violation, which would allow us to include the line number etc.

We should also warn if an allow-comment isn't used and so can be removed.

[ZedThree]

Hmm, this isn't so straight forward to raise it as a normal Diagnostic, as we want a Rule that's not normally checked (i.e. not text, path, or AST).

So maybe we want a fourth category or None -- could also be useful for test rules?

We could continue to hard-error, just with a custom struct with a nicer Display, but this wouldn't work for unused-allow-comment