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/async-function/index.d.ts

@@ -0,0 +1,13 @@
+declare namespace getAsyncFunction {
+    type AsyncFunction<T = unknown> = (...args: any[]) => Promise<T>;
+
+    interface AsyncFunctionConstructor extends FunctionConstructor {
+        new <T>(...args: string[]): AsyncFunction<T>;
+        <T>(...args: string[]): AsyncFunction<T>;
+        readonly prototype: AsyncFunction;
+    }
+}
+
+declare function getAsyncFunction(): getAsyncFunction.AsyncFunctionConstructor | false;
+
+export = getAsyncFunction;