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-browser/test/browser/bootstrap_spec.ts

@@ -269,6 +269,7 @@ export function main() {
            ]
          })
          class SomeModule {
+           ngDoBootstrap() {}
          }
 
          expect(log.result()).toEqual('platform_init1; platform_init2');