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/chunk.js

@@ -0,0 +1,13 @@
+import { slice } from './_setup.js';
+
+// Chunk a single array into multiple arrays, each containing `count` or fewer
+// items.
+export default function chunk(array, count) {
+  if (count == null || count < 1) return [];
+  var result = [];
+  var i = 0, length = array.length;
+  while (i < length) {
+    result.push(slice.call(array, i, i += count));
+  }
+  return result;
+}