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.date.to-iso-string.js

@@ -0,0 +1,10 @@
+'use strict';
+var $ = require('../internals/export');
+var toISOString = require('../internals/date-to-iso-string');
+
+// `Date.prototype.toISOString` method
+// https://tc39.es/ecma262/#sec-date.prototype.toisostring
+// PhantomJS / old WebKit has a broken implementations
+$({ target: 'Date', proto: true, forced: Date.prototype.toISOString !== toISOString }, {
+  toISOString: toISOString
+});