Add typedefs for next.js v7

[mdowds]

Based on the v4 definitions, with new definitions for the new features added since then.

This is my first PR so let me know if there's anything I've missed!

[gantoine]

Thanks for the clean PR, made comparing this with next_v4 super easy!

[ussuritiger]

@mdowds thanks for the typedefs for the Next.js 7. Although after this update I am getting an error while importing getConfig. So

import getConfig from 'next/config';

results into

Cannot resolve module `next/config`.

   1| import getConfig from 'next/config';
                            ^^^^^^^^^^^^^

Any ideas about this?