[FEATURE REQUEST] Allow subclassing of `ReferenceType`

[lucaneg]

Description
The ReferenceType class is used in all sorts of places, making it hard to subclass consistently (e.g., you might create a specific reference type for a language that should be used instead, but the type system still contains instances of ReferenceType).

We should add a method in TypeSystem that produces a reference type, and use that one inside LiSA.