Markdown support

[RDambrosio016]

This is a pretty easy but useful feature, being able to lint markdown files. Implementing this should be quite easy, simply use a regex like

\`\`\`(?:js|javascript)\n([\s\S]*?)\n\`\`\`

Then add a variant to JsFileKind for markdown. Then you can extract each code block using the regex and the first capture group. Then run rules as you would using the src of each group and the same file id. And finally merge the results from each code block run by offsetting the range of each label by the offset of the block. There might be some wonky handling for non-Ok lint runs but overall it should be pretty easy.