NextJS 13 Support

Our starter has been now been updated to Next.js 13!

Test it out with our starter here: https://agilitycms.com/starters/blog-with-nextjs

Source code and docs are here: https://github.com/agility/agilitycms-nextjs-starter

We have updated our Next.js NPM package to support Next 13. You can use this version right now by referencing our Release Candidate here: https://www.npmjs.com/package/@agility/nextjs/v/13.0.0-rc2