Add nix flake

[Yappaholic]

Right now there is no package for Onyx in nixpkgs, so it would be nice to at least have a flake you can use from the main repository.

There is still an option to install Onyx in $HOME/.onyx, but it is not the nix way.

The only thing that can cause trouble is setting up ONYX_PATH, but it can be done with next steps:

# This command returns onyx path in nix-store
nix-store -r $(which onyx)
# Then you can set ONYX_PATH with
export ONYX_PATH=path_to_nix_store