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/internals/is-array.js

@@ -0,0 +1,9 @@
+'use strict';
+var classof = require('../internals/classof-raw');
+
+// `IsArray` abstract operation
+// https://tc39.es/ecma262/#sec-isarray
+// eslint-disable-next-line es/no-array-isarray -- safe
+module.exports = Array.isArray || function isArray(argument) {
+  return classof(argument) === 'Array';
+};