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/_createPredicateIndexFinder.js

@@ -0,0 +1,15 @@
+import cb from './_cb.js';
+import getLength from './_getLength.js';
+
+// Internal function to generate `_.findIndex` and `_.findLastIndex`.
+export default function createPredicateIndexFinder(dir) {
+  return function(array, predicate, context) {
+    predicate = cb(predicate, context);
+    var length = getLength(array);
+    var index = dir > 0 ? 0 : length - 1;
+    for (; index >= 0 && index < length; index += dir) {
+      if (predicate(array[index], index, array)) return index;
+    }
+    return -1;
+  };
+}