* Adds `/flake.nix`, which allows the package to be installed & developed
easily by most NixOS users.
* Adds `/default.nix`, which is an older way that some Nix(OS) users
may expect software to be packaged.
* Adds `/nix/**/*.nix`, which defines various additional helpers used by
`/flake.nix` and `/default.nix`.
* Updates `.gitignore` to include `/result`, which is the default output
location when the package is built using
`nix flake build .` or `nix-build .`
