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/iteratee.js

@@ -0,0 +1,10 @@
+import _ from './underscore.js';
+import baseIteratee from './_baseIteratee.js';
+
+// External wrapper for our callback generator. Users may customize
+// `_.iteratee` if they want additional predicate/iteratee shorthand styles.
+// This abstraction hides the internal-only `argCount` argument.
+export default function iteratee(value, context) {
+  return baseIteratee(value, context, Infinity);
+}
+_.iteratee = iteratee;