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/new-promise-capability.js

@@ -0,0 +1,21 @@
+'use strict';
+var aCallable = require('../internals/a-callable');
+
+var $TypeError = TypeError;
+
+var PromiseCapability = function (C) {
+  var resolve, reject;
+  this.promise = new C(function ($$resolve, $$reject) {
+    if (resolve !== undefined || reject !== undefined) throw new $TypeError('Bad Promise constructor');
+    resolve = $$resolve;
+    reject = $$reject;
+  });
+  this.resolve = aCallable(resolve);
+  this.reject = aCallable(reject);
+};
+
+// `NewPromiseCapability` abstract operation
+// https://tc39.es/ecma262/#sec-newpromisecapability
+module.exports.f = function (C) {
+  return new PromiseCapability(C);
+};