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/internals/object-to-string.js

@@ -0,0 +1,9 @@
+'use strict';
+var TO_STRING_TAG_SUPPORT = require('../internals/to-string-tag-support');
+var classof = require('../internals/classof');
+
+// `Object.prototype.toString` method implementation
+// https://tc39.es/ecma262/#sec-object.prototype.tostring
+module.exports = TO_STRING_TAG_SUPPORT ? {}.toString : function toString() {
+  return '[object ' + classof(this) + ']';
+};