Add mermaid graphs of NLL regions and SCCs to polonius MIR dump This PR expands the polonius MIR dump again with a couple of mermaid charts ported from the graphviz version: - the NLL region graph - and the NLL SCCs I still have done zero visual design on this until now, but [here's](https://gistpreview.github.io/?fbbf900fed2ad21108c7ca0353456398) how it looks (i.e. still bad) just to give an idea of the result. r? `````@matthewjasper````` (feel free to reassign) or anyone