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](https://en.wikipedia.org/wiki/Class_diagram). 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.

Start by reading the existing ClassDiagram stub and the surrounding diagram-generation code. Determine how annotated Pydantic objects and their dependencies should map to UML class diagrams, including layout when classes have multiple dependencies. Done means the implementation can generate a usable class diagram from a Python module with annotated Pydantic objects.

Written by the indexing model from the issue text.