next.js support for on-demand revalidation

Feature request for:
[Data Fetching: Incremental Static Regeneration | Next.js](https://nextjs.org/docs/basic-features/data-fetching/incremental-static-regeneration#on-demand-revalidation-beta)

Technically this is a beta-feature of next, especially notable w/ `unstable_revalidate` method; but I'm wondering if we're considering support for this feature. Especially since traditional ISR doesn't seem to be supported today w/ this project, `on-demand` could be an alternative and perhaps even simpler to implement?

P.S. This project is the most exciting thing coming out of Firebase in a long time, great job!