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/core-js/modules/es.weak-set.constructor.js

@@ -0,0 +1,9 @@
+'use strict';
+var collection = require('../internals/collection');
+var collectionWeak = require('../internals/collection-weak');
+
+// `WeakSet` constructor
+// https://tc39.es/ecma262/#sec-weakset-constructor
+collection('WeakSet', function (init) {
+  return function WeakSet() { return init(this, arguments.length ? arguments[0] : undefined); };
+}, collectionWeak);