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-contain-element.js

@@ -0,0 +1,22 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.toContainElement = toContainElement;
+var _utils = require("./utils");
+function toContainElement(container, element) {
+  (0, _utils.checkHtmlElement)(container, toContainElement, this);
+  if (element !== null) {
+    (0, _utils.checkHtmlElement)(element, toContainElement, this);
+  }
+  return {
+    pass: container.contains(element),
+    message: () => {
+      return [this.utils.matcherHint(`${this.isNot ? '.not' : ''}.toContainElement`, 'element', 'element'), '',
+      // eslint-disable-next-line @babel/new-cap
+      this.utils.RECEIVED_COLOR(`${this.utils.stringify(container.cloneNode(false))} ${this.isNot ? 'contains:' : 'does not contain:'} ${this.utils.stringify(element ? element.cloneNode(false) : element)}
+        `)].join('\n');
+    }
+  };
+}