refactor(core): introduce @NgModule.bootstrap and ngDoBootstrap method

If a `@NgModule` has a `bootstrap` property, `PlatformRef.bootstrapModule` /
`PlatformRef.bootstrapModuleFactory` will automatically bootstrap the components
listed in there.
If such a property does not exist, `PlatformRef.bootstrapModule` /
`PlatformRef.bootstrapModuleFactory` will try to call the method `ngDoBootstrap(appRef: ApplicationRef)` on the module class.
Otherwise an error is reported.

modules/@angular/platform-server/src/server.ts

@@ -110,7 +110,7 @@ export function serverBootstrap<T>(
     providers: customProviders,
     declarations: declarations,
     imports: [BrowserModule],
-    entryComponents: [appComponentType]
+    bootstrap: [appComponentType]
   })
   class DynamicModule {
   }
@@ -121,6 +121,6 @@ export function serverBootstrap<T>(
         const console = moduleRef.injector.get(Console);
         deprecatedConfiguration.deprecationMessages.forEach((msg) => console.warn(msg));
         const appRef: ApplicationRef = moduleRef.injector.get(ApplicationRef);
-        return appRef.bootstrap(appComponentType);
+        return appRef.components[0];
       });
 }