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/define-global-property.js

@@ -0,0 +1,13 @@
+'use strict';
+var globalThis = require('../internals/global-this');
+
+// eslint-disable-next-line es/no-object-defineproperty -- safe
+var defineProperty = Object.defineProperty;
+
+module.exports = function (key, value) {
+  try {
+    defineProperty(globalThis, key, { value: value, configurable: true, writable: true });
+  } catch (error) {
+    globalThis[key] = value;
+  } return value;
+};