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/actual/promise/index.js

@@ -0,0 +1,7 @@
+'use strict';
+var parent = require('../../stable/promise');
+// TODO: Remove from `core-js@4`
+require('../../modules/esnext.promise.try');
+require('../../modules/esnext.promise.with-resolvers');
+
+module.exports = parent;