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/cjs/reject.js

@@ -0,0 +1,10 @@
+var _cb = require('./_cb.js');
+var filter = require('./filter.js');
+var negate = require('./negate.js');
+
+// Return all the elements for which a truth test fails.
+function reject(obj, predicate, context) {
+  return filter(obj, negate(_cb(predicate)), context);
+}
+
+module.exports = reject;