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/full/aggregate-error.js

@@ -0,0 +1,7 @@
+'use strict';
+// TODO: remove from `core-js@4`
+require('../modules/esnext.aggregate-error');
+
+var parent = require('../actual/aggregate-error');
+
+module.exports = parent;