Util digraph source and sink detector

[ChristianMurphy]

In the following graph "Welcome" is a source, because it only has outgoing nodes.
"Web" and "GraphViz!" are sinks because they have only incoming nodes.

digraph G {
  "Welcome" -> "To"
  "To" -> "Web"
  "To" -> "GraphViz!"
}

This proposal would add a isSink and isSource attribute to the data property on edgeStatement nodes.

[ChristianMurphy]

Thanks for starting the discussion @ChristianMurphy !
We're in the process unifying ideas in with discussions unifiedjs/collective#44
If you'd like to continue this thread, or start a new one https://github.com/redotjs/redot/discussions/categories/ideas will be the home for ideas going forward.