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/web/dom-exception.js

@@ -0,0 +1,8 @@
+'use strict';
+require('../modules/es.error.to-string');
+require('../modules/web.dom-exception.constructor');
+require('../modules/web.dom-exception.stack');
+require('../modules/web.dom-exception.to-string-tag');
+var path = require('../internals/path');
+
+module.exports = path.DOMException;