Rollup merge of #136104 - lqd:polonius-debugger-episode-2, r=matthewjasper

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

Author: fmease