Support Next 13 route handlers

[eug-vs]

Next 13 just dropped route handlers for app/ dir, which replace old API routes from pages/ directory. Right now this feature is available on next@canary.

Currently next-auth suggests us to create a pages/api/auth/[...nextauth].ts route, which will handle both UI and API for auth, basically all-in-one. app/ directory will require a new mental model for this, here's how I see it:

• We will need to export GET and POST (and other methods if any) so that consumer can re-use them in route handlers
• Preferably route handlers should use dynamic functions like headers and cookies so that NextJS can can correctly infer when to use dynamic rendering
• The default UI that comes with next-auth should be available separately from route handlers

From the consumer point of view, this could look like this:

// app/api/auth/[...nextauth.ts]/route.ts
import { nextAuthGetHandler, nextAuthPostHandler } from 'next-auth';
import authOptions from '../somewhere/in/project';

export const GET = nextAuthGetHandler(authOptions);
export const POST = nextAuthPostHandler(authOptions);

// app/auth/[...nextauth.ts]/page.tsx
import { NextAuthPage } from 'next-auth';
import authOptions from '../somewhere/in/project';

const Page = NextAuthPage(authOptions);

export default Page;

I wasn't sure if I should open an issue or discussion for this one, please let me know if I need to convert this to an issue instead.