Add ClassDiagram implementation

[kiraparser]

As a programmer, I want to be able to automatically create diagrams from a Python module that has its objects defined and annotated with pydantic. As much as reasonable, I'd want to comply with UML diagrams. This is already a stub implementation of ClassDiagram but it's going to take a lot more effort - because this is no preset ordering of classes, and there may/will be multiple dependencies, the positioning of classes on the page is going to be very different from e.g. a tree diagram.

[ibirothe]

@kiraparser Would you be willing to work on this?

[kiraparser]

Not sure. Recently I've been asking Cursor to generate DrawIO diagrams for me and it actually does a reasonable job - so the immediate pressure is off, TBH.

[ibirothe]

Fair enough. Feel free to share an example class diagram to let us see what kind of representation works for you.