Consider supporting MDX exports

[schickling]

Additionally to defining document properties in frontmatter, MDX also supports defining properties as export statements (see MDX docs). We should consider to support this feature in Contentlayer.

[meleksomai]

@schickling , do you have an idea how to use this? I like to be able to access exports property of an MDX file and be able to use it in the computedFields for instance.

[reubn]

@schickling This is preventing me from moving to Contentlayer

[schickling]

My current idea to address this would be to let the user specify the expected export const myField = ... fields as actual document fields.

I'd love to hear more feedback on this design suggestion!