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/proxy-accessor.js

@@ -0,0 +1,10 @@
+'use strict';
+var defineProperty = require('../internals/object-define-property').f;
+
+module.exports = function (Target, Source, key) {
+  key in Target || defineProperty(Target, key, {
+    configurable: true,
+    get: function () { return Source[key]; },
+    set: function (it) { Source[key] = it; }
+  });
+};