add nix-direnv support; create scripts and configuration for environment management

.envrc

@@ -0,0 +1,5 @@
+if ! has nix_direnv_version || ! nix_direnv_version 3.0.6; then
+    source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.0.6/direnvrc" "sha256-RYcUJaRMf8oF5LznDrlCXbkOQrywm0HDv1VjYGaJGdM="
+fi
+
+use flake

flake.nix

@@ -0,0 +1,31 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    systems.url = "github:nix-systems/default";
+    flake-utils = {
+      url = "github:numtide/flake-utils";
+      inputs.systems.follows = "systems";
+    };
+  };
+
+  outputs =
+    {
+      nixpkgs,
+      flake-utils,
+      ...
+    }:
+    flake-utils.lib.eachDefaultSystem (
+      system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            cargo
+            rustc
+          ];
+        };
+      }
+    );
+}