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/core-js/internals/caller.js

@@ -0,0 +1,8 @@
+'use strict';
+module.exports = function (methodName, numArgs) {
+  return numArgs === 1 ? function (object, arg) {
+    return object[methodName](arg);
+  } : function (object, arg1, arg2) {
+    return object[methodName](arg1, arg2);
+  };
+};