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/@testing-library/jest-dom/dist/to-be-in-the-dom.js

@@ -0,0 +1,22 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.toBeInTheDOM = toBeInTheDOM;
+var _utils = require("./utils");
+function toBeInTheDOM(element, container) {
+  (0, _utils.deprecate)('toBeInTheDOM', 'Please use toBeInTheDocument for searching the entire document and toContainElement for searching a specific container.');
+  if (element) {
+    (0, _utils.checkHtmlElement)(element, toBeInTheDOM, this);
+  }
+  if (container) {
+    (0, _utils.checkHtmlElement)(container, toBeInTheDOM, this);
+  }
+  return {
+    pass: container ? container.contains(element) : !!element,
+    message: () => {
+      return [this.utils.matcherHint(`${this.isNot ? '.not' : ''}.toBeInTheDOM`, 'element', ''), '', 'Received:', `  ${this.utils.printReceived(element ? element.cloneNode(false) : element)}`].join('\n');
+    }
+  };
+}