add super basic nix-darwin configuration for koishi

darwin/koishi/configuration.nix

@@ -0,0 +1,27 @@
+{ pkgs, ... }: {
+ nixpkgs = {
+    config = {
+      allowUnfree = true;
+    };
+  };
+
+  services.nix-daemon.enable = true;
+  nix.settings.experimental-features = "nix-command flakes";
+
+  users.users.lu = {
+    name = "lu";
+    home = "/Users/lu";
+    shell = pkgs.fish;
+  };
+
+  programs.fish = {
+    enable = true;
+    vendor = {
+      completions.enable = true;
+      config.enable = true;
+      functions.enable = true;
+    };
+  };
+
+  system.stateVersion = 5;
+}