fix(@schematics/angular): include default export for Express app

This update is required for Firebase functions compatibility.

Closes #29488

(cherry picked from commit aa0ae45)

Author: alan-agius4

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

@@ -63,3 +63,5 @@ if (isMainModule(import.meta.url)) {
     console.log(`Node Express server listening on http://localhost:${port}`);
   });
 }
+
+export default app;

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

@@ -61,6 +61,6 @@ if (isMainModule(import.meta.url)) {
 }
 
 /**
- * The request handler used by the Angular CLI (dev-server and during build).
+ * Request handler used by the Angular CLI (for dev-server and during build) or Firebase Cloud Functions.
  */
 export const reqHandler = createNodeRequestHandler(app);