Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/workbox-routing/utils/getOrCreateDefaultRouter.js

@@ -0,0 +1,26 @@
+/*
+  Copyright 2019 Google LLC
+
+  Use of this source code is governed by an MIT-style
+  license that can be found in the LICENSE file or at
+  https://opensource.org/licenses/MIT.
+*/
+import { Router } from '../Router.js';
+import '../_version.js';
+let defaultRouter;
+/**
+ * Creates a new, singleton Router instance if one does not exist. If one
+ * does already exist, that instance is returned.
+ *
+ * @private
+ * @return {Router}
+ */
+export const getOrCreateDefaultRouter = () => {
+    if (!defaultRouter) {
+        defaultRouter = new Router();
+        // The helpers that use the default Router assume these listeners exist.
+        defaultRouter.addFetchListener();
+        defaultRouter.addCacheListener();
+    }
+    return defaultRouter;
+};