env: add nix flake and lock files

flake.lock

@@ -0,0 +1,25 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1728018373,
+        "narHash": "sha256-NOiTvBbRLIOe5F6RbHaAh6++BNjsb149fGZd1T4+KBg=",
+        "rev": "bc947f541ae55e999ffdb4013441347d83b00feb",
+        "revCount": 688563,
+        "type": "tarball",
+        "url": "https://api.flakehub.com/f/pinned/NixOS/nixpkgs/0.1.688563%2Brev-bc947f541ae55e999ffdb4013441347d83b00feb/01925b66-3b70-71bd-b277-924a6c208ba7/source.tar.gz"
+      },
+      "original": {
+        "type": "tarball",
+        "url": "https://flakehub.com/f/NixOS/nixpkgs/0.1.%2A.tar.gz"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,25 @@
+{
+  description = "A Nix-flake-based Node.js development environment";
+
+  inputs.nixpkgs.url = "https://flakehub.com/f/NixOS/nixpkgs/0.1.*.tar.gz";
+
+  outputs = { self, nixpkgs }:
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forEachSupportedSystem = f: nixpkgs.lib.genAttrs supportedSystems (system: f {
+        pkgs = import nixpkgs { inherit system; overlays = [ self.overlays.default ]; };
+      });
+    in
+    {
+      overlays.default = final: prev: rec {
+        nodejs = prev.nodejs;
+        yarn = (prev.yarn.override { inherit nodejs; });
+      };
+
+      devShells = forEachSupportedSystem ({ pkgs }: {
+        default = pkgs.mkShell {
+          packages = with pkgs; [ node2nix nodejs nodePackages.pnpm yarn ];
+        };
+      });
+    };
+}