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.array.copy-within.js

@@ -0,0 +1,13 @@
+'use strict';
+var $ = require('../internals/export');
+var copyWithin = require('../internals/array-copy-within');
+var addToUnscopables = require('../internals/add-to-unscopables');
+
+// `Array.prototype.copyWithin` method
+// https://tc39.es/ecma262/#sec-array.prototype.copywithin
+$({ target: 'Array', proto: true }, {
+  copyWithin: copyWithin
+});
+
+// https://tc39.es/ecma262/#sec-array.prototype-@@unscopables
+addToUnscopables('copyWithin');