fix(express-engine): make server.ts compliant with typedef tslint rule

Related to: angular/angular-cli#17834

modules/express-engine/schematics/install/files/__serverFileName@stripTsExtension__.ts

@@ -9,7 +9,7 @@ import { APP_BASE_HREF } from '@angular/common';
 import { existsSync } from 'fs';
 
 // The Express app is exported so that it can be used by serverless Functions.
-export function app() {
+export function app(): void {
   const server = express();
   const distFolder = join(process.cwd(), '<%= browserDistDirectory %>');
   const indexHtml = existsSync(join(distFolder, 'index.original.html')) ? 'index.original.html' : 'index';
@@ -37,7 +37,7 @@ export function app() {
   return server;
 }
 
-function run() {
+function run(): void {
   const port = process.env.PORT || <%= serverPort %>;
 
   // Start up the Node server