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/reject.js

@@ -0,0 +1,8 @@
+import filter from './filter.js';
+import negate from './negate.js';
+import cb from './_cb.js';
+
+// Return all the elements for which a truth test fails.
+export default function reject(obj, predicate, context) {
+  return filter(obj, negate(cb(predicate)), context);
+}