Add dream to nixpkgs for replit support

[ulrikstrid]

replit is a platform for running small playgrounds with web servers. I think they can run "forever" and there might be a option to pay for better performance etc. replit recently added support for using nix, so if we add dream to nixpkgs we can add replit as a playground etc. A node example can be found here

The work of making a nix derivation is mostly done here already so we only have to upstream it when @aantron is comfortable having it upstream.