Feature request

Is your feature request related to a problem? Please describe.

Since I transpile a next.config.ts to next.config.js, it would be good to have typings for the configuration object. This is what is currently typed as any in https://github.com/zeit/next.js/blob/fce7e4eaee45e89557ceaa81fbd42057f8596e22/packages/next-server/server/next-server.ts#L37

This is also a problem when using the programmatic API, since the conf property on the options given to next is typed as any:

import next from "next";
const app = next({
  conf // any
});

Describe the solution you'd like

The previous types provided by @types/next and @types/next-server can be seen here:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/next-server/next-config.d.ts

Perhaps the easiest solution is to type NextConfig with something similar to the version that @types/next-server is based on:
https://github.com/zeit/next.js/blob/7.0.0/server/config.js#L9-L20

Describe alternatives you've considered

The option that I see is to maintain types for the configuration object in parallel to this repository, or simply use the configuration untyped.