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__/LiteralMock.js

@@ -0,0 +1,17 @@
+/**
+ * @flow
+ */
+
+export type LiteralMockType = {|
+  type: 'Literal',
+  value: string,
+  raw: string,
+|};
+
+export default function LiteralMock(value: string): LiteralMockType {
+  return {
+    type: 'Literal',
+    value,
+    raw: value,
+  };
+}