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/internals/is-null-or-undefined.js

@@ -0,0 +1,6 @@
+'use strict';
+// we can't use just `it == null` since of `document.all` special case
+// https://tc39.es/ecma262/#sec-IsHTMLDDA-internal-slot-aec
+module.exports = function (it) {
+  return it === null || it === undefined;
+};