Stubgen support for positional only parameters as per PEP 570

**Feature**

Stubgen should recognize a signature in the shape of `func(param1: T1, param2: T2, /) -> T3` if given in the docstring.

**Pitch**

As https://peps.python.org/pep-0570/ describes, this is common with C extensions. There is currently no way to express these signatures in a way that stubgen can understand and regenerate correctly in the stub. 
