nixos/direnv: init

[infinisil]

Description of changes

nix-direnv is surprisingly annoying to configure correctly, due to it needing both system-wide adjustments (nix.conf changes), but also user-wide adjustments (telling direnv to load nix-direnv using ~/.config/direnv/direnvrc).

This module puts an end to that, allowing a direnv + nix-direnv installation with just

{
  programs.direnv.enable = true;
}

Currently this module comes with nix-direnv enabled by default and there's no way to turn it off. If there ever is a need to only enable direnv without nix-direnv we can do that in another PR. I don't need that myself.

Ping @zimbatm @Mic92 @bbenne10

Things done

• [x] Tested the module