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/eslint-plugin-jsx-a11y/__mocks__/JSXElementMock.js

@@ -0,0 +1,37 @@
+/**
+ * @flow
+ */
+
+import type { JSXAttributeMockType } from './JSXAttributeMock';
+
+export type JSXElementMockType = {
+  type: 'JSXElement',
+  openingElement: {
+    type: 'JSXOpeningElement',
+    name: {
+      type: 'JSXIdentifier',
+      name: string,
+    },
+    attributes: Array<JSXAttributeMockType>,
+  },
+  children: Array<Node>,
+};
+
+export default function JSXElementMock(
+  tagName: string,
+  attributes: Array<JSXAttributeMockType> = [],
+  children?: Array<Node> = [],
+): JSXElementMockType {
+  return {
+    type: 'JSXElement',
+    openingElement: {
+      type: 'JSXOpeningElement',
+      name: {
+        type: 'JSXIdentifier',
+        name: tagName,
+      },
+      attributes,
+    },
+    children,
+  };
+}