Many little improvements

Don't hesitate to review commit by commit.

Further improvements could be made: I think that we could enforce using prettier and jest for example.

{{ cookiecutter.project_slug }}/src/index.tsx

 import React from "react";
-import { render as renderApp } from "react-dom";
+import { createRoot } from "react-dom/client";
 
 import { BrowserRouter, Routes, Route } from "react-router-dom";
 import { Client } from "@cubicweb/client";
@@ -11,7 +11,10 @@
 const client = new Client(`${cwserver}/api`);
 const contextValue = { client, cwserver };
 
-renderApp(
+const rootElement: HTMLElement = document.getElementById("root")!;
+const appRoot = createRoot(rootElement);
+
+appRoot.render(
   <React.StrictMode>
     <CWClientContext.Provider value={contextValue}>
       <BrowserRouter>
@@ -20,6 +23,5 @@
         </Routes>
       </BrowserRouter>
     </CWClientContext.Provider>
-  </React.StrictMode>,
-  document.getElementById("root")
+  </React.StrictMode>
 );