feat(core): introduce @AppModule

Main part for #9726
Closes #9730

modules/@angular/core/src/linker.ts

@@ -7,6 +7,7 @@
  */
 
 // Public API for compiler
+export {AppModuleFactory, AppModuleRef} from './linker/app_module_factory';
 export {Compiler} from './linker/compiler';
 export {ComponentFactory, ComponentRef} from './linker/component_factory';
 export {ComponentFactoryResolver, NoComponentFactoryError} from './linker/component_factory_resolver';