Add support for `--graph=file.mermaid` (#3633)
This PR adds support for mermaid graph syntax (which is also supported
by GitHub markdown).
https://github.blog/2022-02-14-include-diagrams-markdown-files-mermaid/
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Co-authored-by: Greg Soltis <greg@goldfiglabs.com>
Co-authored-by: Greg Soltis <greg.soltis@vercel.com>
