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/flatten.js

@@ -0,0 +1,7 @@
+import _flatten from './_flatten.js';
+
+// Flatten out an array, either recursively (by default), or up to `depth`.
+// Passing `true` or `false` as `depth` means `1` or `Infinity`, respectively.
+export default function flatten(array, depth) {
+  return _flatten(array, depth, false);
+}