lint: consider checking code fence info string

[ee7]

Follow-up to https://github.com/exercism/julia/pull/607

I think a good first step would be to complain about fenced code blocks where the info string is note, caution, or advanced.

The list of languages known to GitHub is here, but I don't immediately know where the source of truth is for the exact list of languages that Exercism can syntax highlight.