Add a healthcheck endpoint to the devserver (#9748)

* Add a healthcheck endpoint to the devserver

This assists in implementing a few self-healing mechanisms.

* Prefix healtcheck endpoint with __parcel

---------

Co-authored-by: pyamada (Remote Dev Environment) <[email protected]>

Author: yamadapc

packages/reporters/dev-server/src/Server.js

@@ -495,6 +495,17 @@ export default class Server {
       setHeaders(res);
       next();
     });
+
+    app.use((req, res, next) => {
+      if (req.url === '/__parcel_healthcheck') {
+        res.statusCode = 200;
+        res.write(`${Date.now()}`);
+        res.end();
+      } else {
+        next();
+      }
+    });
+
     await this.applyProxyTable(app);
     app.use(finalHandler);