Process markdown in computed fields

[seancdavis]

Right now we have a default body field for markdown/mdx files, and we also have a field of type markdown. I'd like to be able to do the same to a computed field, taking advantage of Contentlayer's markdown processing system.

A use case for this is generating an excerpt from the body, but also wanting that excerpt to be in the same structure (Markdown) as the body and any other markdown fields.