feat(flake): modular structure with flake-file and flake-parts

flake/default.nix

@@ -0,0 +1,19 @@
+# Flake
+
+/**
+  Entry point of NixOS configuration
+
+  - Defines external sources and locking their commit, which gives up __purity__.
+  - Declares ready-to-use configurations (e.g. development shells, packages, etc.)
+
+  Essentially, `flake.nix` is like `Cargo.toml` in Rust or `package.json` in Node.js, but for an entire operating system.
+*/
+{ inputs, ... }:
+{
+  imports = [ inputs.flake-file.flakeModules.default ];
+
+  systems = inputs.nixpkgs.lib.systems.flakeExposed;
+
+  flake-file.inputs = import ./inputs;
+  flake-file.outputs = "inputs: inputs.flake-parts.lib.mkFlake { inherit inputs; } ./flake";
+  flake-file.description = "A very basic flake";