Add new d2-based FSM visualization

[mjameswh]

What was changed

• Introduce a new visualization generator for the FSM package.

Why?

• Looks significantly better.
• Easier to generate: cargo generate-fsm-diagrams. Generated files are stored in core/src/worker/workflow/machines/diagrams/svg.

Open Questions and To Do

• Should we gitignore generated files?
• This new visualizer is entirely implemented by the FSM procmacro. In particular, it doesn't feature the "test coverage" aspect of the former puml renderer. It would certainly be possible to port that former logic if we still care about it.
• Cleanup the code. It is more verbose and more intrusive than it should be.

Misc

• Credits to @dandavison