add mermaid support

[s-light]

i like the rendered diagrams from mermaid

would be nice to add mermaid support for this viewer :-)

it seems that the newest/maintained markdown-it plugin i found are
(not listed on the integrations page..)

• markdown-it-textual-uml npm (Aug 31, 2022 | 216 WeeklyDownloads)
• @wekanteam/markdown-it-mermaid - npm (Feb 21, 2023 | 199 WeeklyDownloads)
• @agoose77/markdown-it-mermaid npm (Sep 22, 2022 | 15 WeeklyDownloads)

the other option without plugin seems to be some hacky script embedding...

[Cimbali]

Thanks for suggesting! I was looking at that too.

For the plugin I think we should stick to a really simple one that just adds a class="mermaid" to the graph source code, and then include the mermaid scripts -- as opposed to trying to render the SVG in the markdown machinery.