Add Nix Flake

1. This code doesn't really ever need to be updated (it pulls from the package.json / package-lock.json) 2. It enables two helpful things: 1. `nix develop` drops user's into a shell where everything is installed with exact versions, kind of like docker but without the overhead of a VM 2. `nix build .` is a sandboxed reproducable build. E.g. it prevents "well it works on my machine" kind of bugs This should make for an extremely fast and reliable ZenFS setup from zero: - Install nix: `curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install` - Clone the repo: `nix run 'nixpkgs#git' clone 'git@github.com:zen-fs/core.git'` - Enter the dev shell: `cd core;nix develop` - Run tests: `npm run build; npm run test` The only maintaince is, run `nix flake update` when you want to update the version of node.js that it defaults to.