add dev flake

flake.nix

@@ -0,0 +1,15 @@
+{
+  inputs = {
+    nixpkgs.url = "nixpkgs/nixos-26.05";
+    flakelight = {
+      url = "github:nix-community/flakelight";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+    mill-scale = {
+      url = "git+https://codeberg.org/icewind/mill-scale";
+      inputs.flakelight.follows = "flakelight";
+    };
+  };
+  outputs = {mill-scale, ...}:
+    mill-scale ./. {};
+}