Generator does not detect private constructors

[mateusfccp]

It's a very common pattern to make the default constructor private (like MyObject._(...params)) and provide other public constructors.

I think it would be good if the generator could detect this pattern and generate the copyWith method with the "unamed private constructor".

[numen31337]

I believe this specific case is mentioned in the documentation here.

[mateusfccp]

You are right, I only had looked at the pub.dev page. I think we could include this there too, as it's a common case.