POC for using markdown-it-diagrams

[dmayle]

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This is a hacky POC showing the use of markdown-it-diagrams for mermaid and svgbob. Notable changes:

• Adds svgbob support for ascii diagrams
• CDN config isn't tested because svgbob wasm isn't available via CDN
• Mermaid doesn't show errors. Error code didn't work before removal.

This addresses issue #1549