in development, block server start on warnServer (#22266)

Part of #1029

server.mjs

@@ -58,6 +58,16 @@ async function startServer() {
     if (process.env.HEROKU_PRODUCTION_APP || process.env.GITHUB_ACTIONS) {
       await warmServer()
     }
+  } else {
+    // Warm up as soon as possible when in development.
+    // The `warmServer()` function is idempotent and it will soon be used
+    // by some middleware, but there's no point in having a started server
+    // without this warmed up. Besides, by starting this slow thing now,
+    // it can start immediately instead of waiting for the first request
+    // to trigger it to warm up. That way, when in development and triggering
+    // a `nodemon` restart, there's a good chance the warm up has come some
+    // way before you manage to reach for your browser to do a page refresh.
+    await warmServer()
   }
 
   // Workaround for https://github.com/expressjs/express/issues/1101