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/underscore/modules/chain.js

@@ -0,0 +1,8 @@
+import _ from './underscore.js';
+
+// Start chaining a wrapped Underscore object.
+export default function chain(obj) {
+  var instance = _(obj);
+  instance._chain = true;
+  return instance;
+}