New rules for directives, math, mdx, footnote related

[wooorm]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

There’s a couple syntax extensions we’re pushing and generally supporting across the ecosystem. Some lint rules for those will help.

Solution

• jsx: look through eslint rules?
• lazy lines in footnote definitions

Alternatives

[remcohaszing]

Sure, rules may apply to extension syntax.

I don’t think we should copy ESLint rules. People can use eslint-plugin-mdx to run ESLint rules on MDX files.

[wooorm]

But there are things that eslint cannot do because it doesn’t know about markdown. It probably has some JSX rules that relate to JS. That we can have here as a JSX-relating-to-markdown rule.

[remcohaszing]

Ah yes, that context was missing before. JSX-relating-to-markdown rules make sense.