Simplify DatatypeDeclarationCommand command (#3928)

Simplify DatatypeDeclarationCommand command (#3928)

The new API does not use inheritence for Sorts. The current DatatypeDeclarationCommand uses DatatypeType, which inherits from Type. This commit simplifies the class DatatypeType -> Type and updates the necessary code (e.g. in the printers). Notice we are not yet converting commands Type -> Sort here.

It also makes the main call for constructing datatypes in the parser from DatatypeType -> api::Sort.

This is in preparation for converting Expr-level Datatype to Term-level DatatypeDecl in the parsers.