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-pure/internals/install-error-cause.js

@@ -0,0 +1,11 @@
+'use strict';
+var isObject = require('../internals/is-object');
+var createNonEnumerableProperty = require('../internals/create-non-enumerable-property');
+
+// `InstallErrorCause` abstract operation
+// https://tc39.es/ecma262/#sec-installerrorcause
+module.exports = function (O, options) {
+  if (isObject(options) && 'cause' in options) {
+    createNonEnumerableProperty(O, 'cause', options.cause);
+  }
+};