graphics/d2: Add port

D2 is a text-to-diagram tool, similarly to Mermaid and PlantUML. I find
it far easier to use than PlantUML, and it produces diagrams that--to
my eye--are more visually-appealing than Mermaid. Plus, it's written in
Go, so it can compile diagrams to disk (unlike Mermaid) and doesn't
require a goofy JVM (unlike PlantUML).

https://d2lang.com