feat: nix flake dev environment

Added nix flake dev shell for dev environment configuration.
Added stylua to the dev environment.

Author: fangjunzhou

.envrc

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

.gitignore

@@ -1 +1,2 @@
 /.luarc.json
+.direnv

flake.lock

@@ -0,0 +1,23 @@
+{
+  description = "leetcode.nvim dev environment";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    with flake-utils.lib;
+    eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+        };
+      in
+      {
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs;[ stylua ];
+        };
+      }
+    );
+}