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

packages/compiler-cli/integrationtest/bazel/injectable_def/app/src/dep.ts

@@ -0,0 +1,41 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {Component, Injectable, NgModule} from '@angular/core';
+import {BrowserModule} from '@angular/platform-browser';
+import {ServerModule} from '@angular/platform-server';
+
+@Injectable()
+export class NormalService {
+}
+
+@Component({
+  selector: 'dep-app',
+  template: '{{found}}',
+})
+export class AppComponent {
+  found: boolean;
+  constructor(service: ShakeableService) { this.found = !!service.normal; }
+}
+
+@NgModule({
+  imports: [
+    BrowserModule.withServerTransition({appId: 'id-app'}),
+    ServerModule,
+  ],
+  declarations: [AppComponent],
+  bootstrap: [AppComponent],
+  providers: [NormalService],
+})
+export class DepAppModule {
+}
+
+@Injectable({scope: DepAppModule})
+export class ShakeableService {
+  constructor(readonly normal: NormalService) {}
+}