nixos/doas: init

`doas` is a lighter alternative to `sudo` that "provide[s] 95% of the
features of `sudo` with a fraction of the codebase" [1]. I prefer it to
`sudo`, so I figured I would add a NixOS module in order for it to be
easier to use. The module is based off of the existing `sudo` module.

[1] https://github.com/Duncaen/OpenDoas