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/modules/esnext.array.group.js

@@ -0,0 +1,15 @@
+'use strict';
+var $ = require('../internals/export');
+var $group = require('../internals/array-group');
+var addToUnscopables = require('../internals/add-to-unscopables');
+
+// `Array.prototype.group` method
+// https://github.com/tc39/proposal-array-grouping
+$({ target: 'Array', proto: true }, {
+  group: function group(callbackfn /* , thisArg */) {
+    var thisArg = arguments.length > 1 ? arguments[1] : undefined;
+    return $group(this, callbackfn, thisArg);
+  }
+});
+
+addToUnscopables('group');