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/es.object.assign.js

@@ -0,0 +1,10 @@
+'use strict';
+var $ = require('../internals/export');
+var assign = require('../internals/object-assign');
+
+// `Object.assign` method
+// https://tc39.es/ecma262/#sec-object.assign
+// eslint-disable-next-line es/no-object-assign -- required for testing
+$({ target: 'Object', stat: true, arity: 2, forced: Object.assign !== assign }, {
+  assign: assign
+});