fix(core): export inject() from @angular/core (#22389)

inject() supports the ngInjectableDef-based configuration of the injector
(otherwise known as tree-shakeable services). It was missing from the
exported API of @angular/core, this PR adds it.

The test added here is correct in theory, but may pass accidentally due
to the decorator side-effect replacing the inject() call at runtime. An
upcoming compiler PR will strip reified decorators from the output
entirely.

Fixes #22388

PR Close #22389

tools/public_api_guard/core/core.d.ts

@@ -447,6 +447,9 @@ export interface HostDecorator {
 /** @stable */
 export declare const HostListener: HostListenerDecorator;
 
+/** @experimental */
+export declare function inject<T>(token: Type<T> | InjectionToken<T>, notFoundValue?: undefined, flags?: InjectFlags): T;
+
 /** @stable */
 export declare const Inject: InjectDecorator;