add a nix run '.#clean' rule

README.md

@@ -19,4 +19,6 @@ roughly:
 2. `tree-sitter generate`
 3. `tree-sitter test`
 
+run `nix run '.#clean'` before committing to avoid checking in build artifacts.
+
 the parser is defined in `grammar.js`, and tests live in `corpus/`.

flake.nix

@@ -16,5 +16,14 @@
       defaultPackage = packages.tree-sitter-nixshell;
 
       devShells.default = pkgs.callPackage ./shell.nix { };
+
+      apps.clean = {
+        type = "app";
+        program = builtins.toString (pkgs.writeShellScript "clean" ''
+          # remove build artifacts
+          rm -f Cargo.toml binding.gyp
+          rm -rf bindings/ src/
+        '');
+      };
     });
 }