added nix flake

flake.nix

@@ -0,0 +1,30 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs = { self, nixpkgs, ... } @ inputs:
+    let
+      pkgs = nixpkgs.legacyPackages.x86_64-linux;
+    in
+    {
+      devShells.x86_64-linux.default = pkgs.mkShell {
+        buildInputs = [
+          (pkgs.lua5_1.withPackages (ps: with ps; [
+            (
+              let
+                buildLuarocksPackage = ps.buildLuarocksPackage;
+              in
+              pkgs.callPackage ./cmark.nix {
+                inherit buildLuarocksPackage;
+              }
+            )
+            luafilesystem
+            luasocket
+            luasec
+            fennel
+          ]))
+        ];
+      };
+    };
+}