feat(nix-flake): add nix flake and nix module

nix/shell.nix

@@ -0,0 +1,24 @@
+{ pkgs ? (
+    let
+      inherit (builtins) fetchTree fromJSON readFile;
+      inherit ((fromJSON (readFile ./flake.lock)).nodes) nixpkgs gomod2nix;
+    in
+    import (fetchTree nixpkgs.locked) {
+      overlays = [
+        (import "${fetchTree gomod2nix.locked}/overlay.nix")
+      ];
+    }
+  )
+, mkGoEnv ? pkgs.mkGoEnv
+, gomod2nix ? pkgs.gomod2nix
+}:
+
+let
+  goEnv = mkGoEnv { pwd = ../.; };
+in
+pkgs.mkShell {
+  packages = [
+    goEnv
+    gomod2nix
+  ];
+}