Support for set type attributes

[ivan-alfonso]

Adding support for ASTs with Set type parameters to ASTTransformer would be valuable. For example, have the ability to address the following metamodel (with a Set type parameter) in a transformation.

@dataclass
class Library(Node):
    books: Set["Book"] = field(default_factory=set)

@dataclass
class Book(Node):
    name: str = ""

[ftomassetti]

Thank you fore reporting this @ivan-alfonso !