Disallow missing label references.
Markdown allows you to specify a label as a placeholder for a URL in both links and images using square brackets, such as:
[ESLint][eslint]
[eslint]: https://eslint.org
If the label is never defined, then Markdown doesn't render a link and instead renders plain text.
This rule warns when it finds text that looks like it's a label but the label reference doesn't exist.
Examples of incorrect code:
[ESLint][eslint]
[eslint][]
[eslint]
If you aren't concerned with missing label references, you can safely disable this rule.