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/is-callable.js

@@ -0,0 +1,12 @@
+'use strict';
+// https://tc39.es/ecma262/#sec-IsHTMLDDA-internal-slot
+var documentAll = typeof document == 'object' && document.all;
+
+// `IsCallable` abstract operation
+// https://tc39.es/ecma262/#sec-iscallable
+// eslint-disable-next-line unicorn/no-typeof-undefined -- required for testing
+module.exports = typeof documentAll == 'undefined' && documentAll !== undefined ? function (argument) {
+  return typeof argument == 'function' || argument === documentAll;
+} : function (argument) {
+  return typeof argument == 'function';
+};