Add Nix flake and direnv files.

.envrc

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+
+use flake

flake.nix

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+{
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = import ./shell.nix { inherit pkgs; };
+      }
+    );
+}

shell.nix

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+{ pkgs ? import <nixpkgs> { } }:
+
+with pkgs;
+
+mkShell rec {
+  packages = [ cargo-make nodejs nodePackages.pnpm ];
+}