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/cjs/propertyOf.js

@@ -0,0 +1,12 @@
+var get = require('./get.js');
+var noop = require('./noop.js');
+
+// Generates a function for a given object that returns a given property.
+function propertyOf(obj) {
+  if (obj == null) return noop;
+  return function(path) {
+    return get(obj, path);
+  };
+}
+
+module.exports = propertyOf;