Proper check for first-class types in datatype subfields (#7712)

Proper check for first-class types in datatype subfields (#7712)

Fixes two issues with our check for datatype subfields (isFunctionLike -> isFirstClass): functions should be allowed, RegLan should not.

Fixes cvc5/cvc5-projects#368. That issue now throws an error:

(error "Parse Error: proj-368.smt2:3.39: cannot use fields in datatypes that are not first class types

  (declare-datatype x ((_x (x8 RegLan))))
                                         ^
")
This adds a regression showing we allow functions as subfields.