go: add template for golang

flake.nix

···
           description = "generic nix flake";
         };
       }
-      // nixpkgs.lib.genAttrs [ "erlang" "python" ] (language: {
+      // nixpkgs.lib.genAttrs [ "erlang" "go" "python" ] (language: {
         path = ./${language};
         description = "${language} nix flake";
       });

go/flake.nix

···
+{
+  description = "TODO: Describe the flake";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
+
+  outputs =
+    { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" ];
+      forSystems = func: nixpkgs.lib.genAttrs systems (system: func (import nixpkgs { inherit system; }));
+    in
+    {
+      formatter = forSystems (pkgs: pkgs.nixfmt-tree.override { settings.formatter.nixfmt.options = [ "-w 120" ]; });
+
+      devShells = forSystems (pkgs: {
+        default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            go
+            gopls
+            gotools
+          ];
+        };
+      });
+    };
+}