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/underscore/modules/isNaN.js

@@ -0,0 +1,7 @@
+import { _isNaN } from './_setup.js';
+import isNumber from './isNumber.js';
+
+// Is the given value `NaN`?
+export default function isNaN(obj) {
+  return isNumber(obj) && _isNaN(obj);
+}