Add ClassDiagram implementation

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.