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/react-dev-utils/getCacheIdentifier.js

@@ -0,0 +1,21 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+module.exports = function getCacheIdentifier(environment, packages) {
+  let cacheIdentifier = environment == null ? '' : environment.toString();
+  for (const packageName of packages) {
+    cacheIdentifier += `:${packageName}@`;
+    try {
+      cacheIdentifier += require(`${packageName}/package.json`).version;
+    } catch (_) {
+      // ignored
+    }
+  }
+  return cacheIdentifier;
+};