Prohibit user-defined parametric sorts

[Scott-Guest]

This PR adds a check to error on any non-MInt parametric sorts.

We already assume MInt is the only parametric sort internally, so this check just makes that explicit to the user:

k/kernel/src/main/java/org/kframework/parser/inner/RuleGrammarGenerator.java

Line 419 in 0b4353c

// TODO: for now we only have `MInt{Widht}` as parametric sorts, so we can treat it as a corner

The check has to be done in Module.checkSorts so that we error early enough, e.g., before computing the subsort POSet.