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/modules/es.promise.reject.js

@@ -0,0 +1,15 @@
+'use strict';
+var $ = require('../internals/export');
+var newPromiseCapabilityModule = require('../internals/new-promise-capability');
+var FORCED_PROMISE_CONSTRUCTOR = require('../internals/promise-constructor-detection').CONSTRUCTOR;
+
+// `Promise.reject` method
+// https://tc39.es/ecma262/#sec-promise.reject
+$({ target: 'Promise', stat: true, forced: FORCED_PROMISE_CONSTRUCTOR }, {
+  reject: function reject(r) {
+    var capability = newPromiseCapabilityModule.f(this);
+    var capabilityReject = capability.reject;
+    capabilityReject(r);
+    return capability.promise;
+  }
+});