Support for set type attributes

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.

```python
@dataclass
class Library(Node):
    books: Set["Book"] = field(default_factory=set)

@dataclass
class Book(Node):
    name: str = ""
```