Add support for rendering circular diagrams

[str8edgedave]

Add support for a circular diagram to show a process that does not have a start or end point.

See https://www.conceptdraw.com/How-To-Guide/circular-diagram for example.

[str8edgedave]

For example, the following code generates a long vertical chain, with an arrow pointing back to the top.

$ cat test.d2 step-1 step-1 -> step-2 step-2 -> step-3 step-3 -> step-4 step-4 -> step-5 step-5 -> step-1

I realize that D2 can use different rendering engines, so I'm not sure how possible this request is...

[alixander]

hey @str8edgedave thanks for the request, it looks like a feature request for D2 so I've moved it to another repo (the text-to-diagram one is specifically for comparisons)