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/amd/isNaN.js

@@ -0,0 +1,10 @@
+define(['./_setup', './isNumber'], function (_setup, isNumber) {
+
+  // Is the given value `NaN`?
+  function isNaN(obj) {
+    return isNumber(obj) && _setup._isNaN(obj);
+  }
+
+  return isNaN;
+
+});