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-empty-dom-element.js

@@ -0,0 +1,29 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+exports.toBeEmptyDOMElement = toBeEmptyDOMElement;
+var _utils = require("./utils");
+function toBeEmptyDOMElement(element) {
+  (0, _utils.checkHtmlElement)(element, toBeEmptyDOMElement, this);
+  return {
+    pass: isEmptyElement(element),
+    message: () => {
+      return [this.utils.matcherHint(`${this.isNot ? '.not' : ''}.toBeEmptyDOMElement`, 'element', ''), '', 'Received:', `  ${this.utils.printReceived(element.innerHTML)}`].join('\n');
+    }
+  };
+}
+
+/**
+ * Identifies if an element doesn't contain child nodes (excluding comments)
+ * ℹ Node.COMMENT_NODE can't be used because of the following issue 
+ * https://github.com/jsdom/jsdom/issues/2220
+ *
+ * @param {*} element an HtmlElement or SVGElement
+ * @return {*} true if the element only contains comments or none
+ */
+function isEmptyElement(element) {
+  const nonCommentChildNodes = [...element.childNodes].filter(node => node.nodeType !== 8);
+  return nonCommentChildNodes.length === 0;
+}