Add Next browser

[Chris Warburton]

Conkeror seems to have died because of Mozilla's overhauls of
XUL/Gecko/Quantum/etc. Looks like "next-browser" is the spiritual
successor:

https://github.com/next-browser/next/tree/master/next#linux-setup-experimental

Looks like any package will require some common lisp stuff, which I
don't yet know how to do ;)

Perhaps some other browser would be preferable? Or a Firefox plugin?

NOTE: This was copied over from nix-config