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.set.union.v2.js

@@ -0,0 +1,13 @@
+'use strict';
+var $ = require('../internals/export');
+var union = require('../internals/set-union');
+var setMethodGetKeysBeforeCloning = require('../internals/set-method-get-keys-before-cloning-detection');
+var setMethodAcceptSetLike = require('../internals/set-method-accept-set-like');
+
+var FORCED = !setMethodAcceptSetLike('union') || !setMethodGetKeysBeforeCloning('union');
+
+// `Set.prototype.union` method
+// https://tc39.es/ecma262/#sec-set.prototype.union
+$({ target: 'Set', proto: true, real: true, forced: FORCED }, {
+  union: union
+});