refactor(@schematics/angular): Out of the box support for PM2

Because PM2 is the most popular node process manager, it makes sense to
support it out of the box.

Fixes angular#31081

Author: PowerKiKi

packages/schematics/angular/ssr/files/application-builder/server.ts.template

@@ -48,10 +48,10 @@ app.use((req, res, next) => {
 });
 
 /**
- * Start the server if this module is the main entry point.
+ * Start the server if this module is the main entry point, or it is ran via PM2.
  * The server listens on the port defined by the `PORT` environment variable, or defaults to 4000.
  */
-if (isMainModule(import.meta.url)) {
+if (isMainModule(import.meta.url) || process.env.pm_id) {
   const port = process.env['PORT'] || 4000;
   app.listen(port, (error) => {
     if (error) {