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/_optimizeCb.js

@@ -0,0 +1,23 @@
+// Internal function that returns an efficient (for current engines) version
+// of the passed-in callback, to be repeatedly applied in other Underscore
+// functions.
+function optimizeCb(func, context, argCount) {
+  if (context === void 0) return func;
+  switch (argCount == null ? 3 : argCount) {
+    case 1: return function(value) {
+      return func.call(context, value);
+    };
+    // The 2-argument case is omitted because we’re not using it.
+    case 3: return function(value, index, collection) {
+      return func.call(context, value, index, collection);
+    };
+    case 4: return function(accumulator, value, index, collection) {
+      return func.call(context, accumulator, value, index, collection);
+    };
+  }
+  return function() {
+    return func.apply(context, arguments);
+  };
+}
+
+module.exports = optimizeCb;