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-pure/modules/esnext.weak-set.of.js

@@ -0,0 +1,10 @@
+'use strict';
+var $ = require('../internals/export');
+var WeakSetHelpers = require('../internals/weak-set-helpers');
+var createCollectionOf = require('../internals/collection-of');
+
+// `WeakSet.of` method
+// https://tc39.github.io/proposal-setmap-offrom/#sec-weakset.of
+$({ target: 'WeakSet', stat: true, forced: true }, {
+  of: createCollectionOf(WeakSetHelpers.WeakSet, WeakSetHelpers.add, false)
+});