ESLint Markdown plugins

[Mitsunee]

• https://github.com/eslint/markdown
• https://github.com/mdx-js/eslint-mdx

not sure if the markdown plugin can be used with mdx, tho I would like some of the rules from that if possible

[Mitsunee]

eslint/markdown may support mdx in the future, so I'm delaying this until there's a decision on that: eslint/markdown#316