Support linting JavaScript snippets in MarkDown documentation

[ericcornelissen]

Summary

Add support for linting JavaScript snippets inside of MarkDown documentation in order to catch problematic regular expressions advertised in documentation. Regular expression appearing in documentation might be copied by readers and thus end up in code. As such, problematic regular expression in documentation should be reported on, to avoid such regular expression ending up in code (that isn't being scanned for problematic regular expression).

It should be possible to provide support for this using eslint-plugin-markdown.