Better Gatsby Support

[ricokahler]

If there's enough interested, I'd love to collaborate on a better Gatsby solution than Gatsby's Gatsby without GraphQL guide.

Though this works, it's less than ideal because Gatsby won't officially support gatsby-node in typescript.

This is an issue because the preferred way to use the sanity-codegen client is to query for a whole document and do projections and transforms in code. However if you have to populate the Gatsby context using JavaScript only, you loose on some type inference opportunities.

A better solution would be a Sanity Codegen Gatsby source plugin. I'm not currently working on a Gatsby project so I'd rather collaborate with one of y'all.

Let me know if interested!