[plugins] Support Pikchr Diagrams

[benrbray]

It would be cool to support Pikchr diagrams as an editor plugin.

Pikchr (pronounced "picture") is a PIC-like markup language for diagrams in technical documentation. Pikchr is designed to be embedded in fenced code blocks of Markdown or similar mechanisms of other documentation markup languages.

This might be an interesting case study for testing the developer experience of writing a third-party plugin, as it would involve:

• The inclusion of a third-party library
• A modification to how an existing syntax element (code blocks) is handled
• A complicated postprocessor on the Pikchr syntax that renders the code block as an image

Questions:

• Does Pikchr offer a way to render math syntax inside of a diagram?