Switch to github.com/yuin/goldmark library for all Markdown processing

[SBGoods]

Supersedes: #131

This library currently uses the russross/blackfriday library to implement a custom Markdown renderer for the plainmarkdown template function. blackfriday/v1 lacks an exposed AST for parsing and blackfriday/v2 lacks support/extensions for pure Markdown parsing and rendering.

The migrate subcommand currently uses goldmark for Markdown parsing but the mdplain package used by the generate subcommand will need to be migrated to goldmark

[github-actions]

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