refactor: Tidy flake.nix and break it apart

modules/nixos/defaults/nixpkgs.nix

@@ -0,0 +1,20 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let cfg = config.sydnix.defaults.nixpkgs;
+in {
+  options = {
+    sydnix.defaults.nixpkgs = {
+      enable = mkOption {
+        description = "Madeleine's default Nixpkgs settings";
+        default = true;
+        type = types.bool;
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    nixpkgs.config.allowUnfree = true;
+  };
+}