Add Nix Flake

[a-camarillo]

Having a nix flake won't do much other than create a dev shell that will guarantee that anyone who wants to contribute to the project can work in the same dev environment as everyone else. Packaging is similar. It's a guarantee. Also the build will be sandboxed so that you can do package releases on GitHub or anyone who wants to build it themselves can do so. And it will be exactly the same for everyone.