Add nixpkgs and easy-purescript-nix as easily updateable niv sources

[tfc]

This PR does:

• In order to make it easier to update the shell.nix environment, i added a niv sources.json file.
  • This way one can simply run niv update && git add nix/sources.json && git commit -m "Updated nix inputs" from time to time to keep the shell.nix up to date.
  • not only easy-purescript-nix is pinned now, but also the nixpkgs input (which in turn pins nodejs and all the other involved tools).
• In addition to that, i added the nodejs input because it is a dependency when running the makefile targets.

I have compiled some examples with that, and it works great.