Nix flake

[TWB0109]

Created a nix flake for this project so people can easily run it from nixos or nix on a different system with flakes enabled.

Here's a picture of how a Nix or Nixos user would run this (from my branch)

If this were merged, it would be run like this:

nix run github:chmin/nvim-ctrl

I'm not that good at nix, and most of this was copied from zero-to-nix, so I can't promise maintenance until I become better at the language. I'd understand if this can't be merged, but I figured I'd create the PR JIC.