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/mixin.js

@@ -0,0 +1,18 @@
+define(['./_setup', './underscore', './_chainResult', './each', './functions'], function (_setup, underscore, _chainResult, each, functions) {
+
+  // Add your own custom functions to the Underscore object.
+  function mixin(obj) {
+    each(functions(obj), function(name) {
+      var func = underscore[name] = obj[name];
+      underscore.prototype[name] = function() {
+        var args = [this._wrapped];
+        _setup.push.apply(args, arguments);
+        return _chainResult(this, func.apply(underscore, args));
+      };
+    });
+    return underscore;
+  }
+
+  return mixin;
+
+});