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/compiler-cli/integrationtest/src/module.ts

@@ -34,4 +34,6 @@ import {CompWithChildQuery, CompWithDirectiveChild} from './queries';
 })
 export class MainModule {
   constructor(public appRef: ApplicationRef) {}
+
+  ngDoBootstrap() {}
 }