Add support for Next.js app router exports

[bradlc]

Ref: markdoc/markdoc#416, markdoc/markdoc#418

This PR adds support for specifying the metadata and revalidate exports when using app router:

---
nextjs:
  metadata:
    title: Lorem ipsum
    description: Lorem ipsum dolor sit amet
  revalidate: 60
---

Closes markdoc/markdoc#418
Closes markdoc/markdoc#416

[mfix-stripe]

@bradlc I'd like to group all the Next.js specific items under a single key, as you suggested above. I was thinking the key could be called next.js or nextjs, just to disambiguate with the work "next".

What do you think?

[bradlc]

Makes sense! I've updated the PR to include all exports that I could find under a nextjs key.

Export names come from:

• https://nextjs.org/docs/app/api-reference/file-conventions/route-segment-config
• https://nextjs.org/docs/app/building-your-application/optimizing/metadata

Let me know if you have any additional feedback! 🤙

[mfix-stripe]

Thanks @bradlc! Just a couple comments

[mfix-stripe]

@rpaul-stripe WDYT about using the nextjs frontmatter key to allow users to specify Next.js specific features (like metadata and revalidate)

Alternative keys:

• next.js
• config / options (higher chance for collisions)

I think I like next.js best (personally)

[rpaul-stripe]

I think nextjs is good.

[mfix-stripe]

@rpaul-stripe WDYT about using the nextjs frontmatter key to allow users to specify Next.js specific features (like metadata and revalidate)

Alternative keys:

• next.js
• config / options (higher chance for collisions)

I think I like next.js best (personally)