Add .jsx filename extension

(This is also required for Vite later)

web/src/index.jsx

@@ -0,0 +1,6 @@
+import * as React from "react";
+import { createRoot } from "react-dom/client";
+import App from "./components/App";
+
+const root = createRoot(document.querySelector("#root"));
+root.render(<App />);