feat(core): Introduce TestBed.inject to replace TestBed.get (#32200)

TestBed.get is not type safe, fixing it would be a massive breaking
change. The Angular team has proposed replacing it with TestBed.inject
and deprecate TestBed.get.

Deprecation from TestBed.get will come as a separate commit.

Issue #26491
Fixes #29905

BREAKING CHANGE: Injector.get now accepts abstract classes to return
type-safe values. Previous implementation returned `any` through the
deprecated implementation.

PR Close #32200

aio/src/app/app.component.spec.ts

@@ -677,7 +677,7 @@ describe('AppComponent', () => {
       });
 
       it('should not be loaded/registered until necessary', () => {
-        const loader: TestElementsLoader = fixture.debugElement.injector.get(ElementsLoader);
+        const loader = fixture.debugElement.injector.get(ElementsLoader) as unknown as TestElementsLoader;
         expect(loader.loadCustomElement).not.toHaveBeenCalled();
 
         setHasFloatingToc(true);