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/ie8-dom-define.js

@@ -0,0 +1,12 @@
+'use strict';
+var DESCRIPTORS = require('../internals/descriptors');
+var fails = require('../internals/fails');
+var createElement = require('../internals/document-create-element');
+
+// Thanks to IE8 for its funny defineProperty
+module.exports = !DESCRIPTORS && !fails(function () {
+  // eslint-disable-next-line es/no-object-defineproperty -- required for testing
+  return Object.defineProperty(createElement('div'), 'a', {
+    get: function () { return 7; }
+  }).a !== 7;
+});