fix: update template files for Node.js libraries (#463)

synthtool/gcp/templates/node_library/.eslintrc.json

@@ -0,0 +1,3 @@
+{
+  "extends": "./node_modules/gts"
+}

synthtool/gcp/templates/node_library/.prettierrc.js

@@ -0,0 +1,17 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+module.exports = {
+  ...require('gts/.prettierrc.json')
+}