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/isArguments.js

@@ -0,0 +1,16 @@
+import tagTester from './_tagTester.js';
+import has from './_has.js';
+
+var isArguments = tagTester('Arguments');
+
+// Define a fallback version of the method in browsers (ahem, IE < 9), where
+// there isn't any inspectable "Arguments" type.
+(function() {
+  if (!isArguments(arguments)) {
+    isArguments = function(obj) {
+      return has(obj, 'callee');
+    };
+  }
+}());
+
+export default isArguments;