Query for code inside markdown files

[morgante]

For many use cases, it's valuable to also migrate code inside markdown blocks.

Example case: https://github.com/jxnl/instructor/pull/546/files#diff-ee74b0e63f6efc4ae309c8951c5678e305ff9f017f3ce9b0c3f785d67e857e9e

We should be able to use the same approach we use for Vue and other "embedded" languages.