feat: allow direct scoping of @Injectables to the root injector (#22185)

@Injectable() supports a scope parameter which specifies the target module.
However, it's still difficult to specify that a particular service belongs
in the root injector. A developer attempting to ensure that must either
also provide a module intended for placement in the root injector or target
a module known to already be in the root injector (e.g. BrowserModule).
Both of these strategies are cumbersome and brittle.

Instead, this commit adds a token APP_ROOT_SCOPE which provides a
straightforward way of targeting the root injector directly, without
requiring special knowledge of modules within it.

PR Close #22185

packages/core/src/di.ts

@@ -23,3 +23,4 @@ export {StaticProvider, ValueProvider, ExistingProvider, FactoryProvider, Provid
 export {ResolvedReflectiveFactory, ResolvedReflectiveProvider} from './di/reflective_provider';
 export {ReflectiveKey} from './di/reflective_key';
 export {InjectionToken} from './di/injection_token';
+export {APP_ROOT_SCOPE} from './di/scope';