chore(nix): add flake with devShell and direnv support

.envrc

@@ -0,0 +1 @@
+use flake

.gitignore

@@ -5,3 +5,5 @@
 # These are backup files generated by rustfmt
 **/*.rs.bk
 
+# Ignore direnv directory
+.direnv/

flake.nix

@@ -0,0 +1,44 @@
+{
+  description = "Development environment for commit-email";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      systems = [
+        "x86_64-linux"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "aarch64-darwin"
+      ];
+      forAllSystems = nixpkgs.lib.genAttrs systems;
+    in
+    {
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+        in
+        {
+          default = pkgs.mkShell {
+            packages = with pkgs; [
+              rustc
+              rustfmt
+              cargo
+              clippy
+
+              # Required by openssl-sys crate
+              pkg-config
+              openssl
+
+              nil
+              nixfmt
+            ];
+          };
+        }
+      );
+    };
+}