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/_baseIteratee.js

@@ -0,0 +1,17 @@
+import identity from './identity.js';
+import isFunction from './isFunction.js';
+import isObject from './isObject.js';
+import isArray from './isArray.js';
+import matcher from './matcher.js';
+import property from './property.js';
+import optimizeCb from './_optimizeCb.js';
+
+// An internal function to generate callbacks that can be applied to each
+// element in a collection, returning the desired result — either `_.identity`,
+// an arbitrary callback, a property matcher, or a property accessor.
+export default function baseIteratee(value, context, argCount) {
+  if (value == null) return identity;
+  if (isFunction(value)) return optimizeCb(value, context, argCount);
+  if (isObject(value) && !isArray(value)) return matcher(value);
+  return property(value);
+}