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/esnext.weak-set.add-all.js

@@ -0,0 +1,15 @@
+'use strict';
+var $ = require('../internals/export');
+var aWeakSet = require('../internals/a-weak-set');
+var add = require('../internals/weak-set-helpers').add;
+
+// `WeakSet.prototype.addAll` method
+// https://github.com/tc39/proposal-collection-methods
+$({ target: 'WeakSet', proto: true, real: true, forced: true }, {
+  addAll: function addAll(/* ...elements */) {
+    var set = aWeakSet(this);
+    for (var k = 0, len = arguments.length; k < len; k++) {
+      add(set, arguments[k]);
+    } return set;
+  }
+});