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/define-built-in-accessor.js

@@ -0,0 +1,9 @@
+'use strict';
+var makeBuiltIn = require('../internals/make-built-in');
+var defineProperty = require('../internals/object-define-property');
+
+module.exports = function (target, name, descriptor) {
+  if (descriptor.get) makeBuiltIn(descriptor.get, name, { getter: true });
+  if (descriptor.set) makeBuiltIn(descriptor.set, name, { setter: true });
+  return defineProperty.f(target, name, descriptor);
+};