add simple direnv support

[nim65s]

Hi,

This add a simplified direnv module.

It is good enough for my use cases, but I have no idea if this is in the scope of this project or if I should do this elsewhere.

fix #199

[zimbatm]

Nice, thank you.

One idea we had was that system-manager should expose enough NixOS-like stubs that it becomes possible to import modules from NixOS directly. The coverage isn't complete right now, but maybe imports = [ "${nixpkgs}/nixos/modules/programs/direnv.nix" ] works?

Alternatively, we could also merge system-manager modules, but then we should probably use some sort of prefix so they don't clash with NixOS modules. Something short like sm..

[nim65s]

Great, thanks for the answer !

I can't test that for the next 2 weeks, so if anybody want to give it a try please don't hesitate.

Otherwise, I'll get back to this in July :)

[r-vdp]

Did this work out with the upstream module in the end, or do we still want to merge this PR?