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/require-object-coercible.js

@@ -0,0 +1,11 @@
+'use strict';
+var isNullOrUndefined = require('../internals/is-null-or-undefined');
+
+var $TypeError = TypeError;
+
+// `RequireObjectCoercible` abstract operation
+// https://tc39.es/ecma262/#sec-requireobjectcoercible
+module.exports = function (it) {
+  if (isNullOrUndefined(it)) throw new $TypeError("Can't call method on " + it);
+  return it;
+};