feature: Customize `Visitor.decorators_to_labels`

[mpharrigan]

I'd like to customize the behavior of Visitor.decorators_to_labels (my specific use case is that I want to handle @propertys as methods instead of attributes). It seems that there isn't an easy extension point to modify the behavior of this method.

I can monkeypatch the builtin_decorators and stdlib_decorators global mappings in visitor.py, but this could be brittle