feat(router): deprecate loadChildren:string (#30073)

The proposed ES dynamic import() is now supported by the Angular CLI and the
larger toolchain. This renders the `loadChildren: string` API largely
redundant, as import() is far more natural, is less error-prone, and is
standards compliant. This commit deprecates the `string` form of
`loadChildren` in favor of dynamic import().

DEPRECATION:

When defining lazy-loaded route, Angular previously offered two options for
configuring the module to be loaded, both via the `loadChildren` parameter
of the route. Most Angular developers are familiar withthe `string` form of
this API. For example, the following route definition configures Angular to
load a `LazyModule` NgModule from `lazy-route/lazy.module.ts`:

```
[{
  path: 'lazy',
  loadChildren: 'lazy-route/lazy.module#LazyModule',
}]
```

This "magic string" configuration was previously necessary as there was
no dynamic module loading standard on the web. This has changed with the
pending standardization of dynamic `import()` expressions, which are now
supported in the Angular CLI and in web tooling in general. `import()`
offers a more natural and robust solution to dynamic module loading. The
above example can be rewritten to use dynamic `import()`:

```
[{
  path: 'lazy',
  loadChildren: () => import('./lazy-route/lazy.module').then(mod => mod.LazyModule),
}]
```

This form of lazy loading offers significant advantages in terms of:

* type checking via TypeScript
* simplicity of generated code
* future potential to run natively in supporting browsers
  (see: [caniuse: dynamic import()](https://caniuse.com/#feat=es6-module-dynamic-import))

As a result, Angular is deprecating the `loadChildren: string` syntax in
favor of ES dynamic `import()`. An automatic migration will run during
`ng upgrade` to convert your existing Angular code to the new syntax.

PR Close #30073

tools/public_api_guard/core/core.d.ts

@@ -586,6 +586,7 @@ export declare abstract class NgModuleFactory<T> {
     abstract create(parentInjector: Injector | null): NgModuleRef<T>;
 }
 
+/** @deprecated */
 export declare abstract class NgModuleFactoryLoader {
     abstract load(path: string): Promise<NgModuleFactory<any>>;
 }
@@ -1297,11 +1298,13 @@ export interface SkipSelfDecorator {
 
 export declare type StaticProvider = ValueProvider | ExistingProvider | StaticClassProvider | ConstructorProvider | FactoryProvider | any[];
 
+/** @deprecated */
 export declare class SystemJsNgModuleLoader implements NgModuleFactoryLoader {
     constructor(_compiler: Compiler, config?: SystemJsNgModuleLoaderConfig);
     load(path: string): Promise<NgModuleFactory<any>>;
 }
 
+/** @deprecated */
 export declare abstract class SystemJsNgModuleLoaderConfig {
     factoryPathPrefix: string;
     factoryPathSuffix: string;

tools/public_api_guard/router/router.d.ts

@@ -101,6 +101,9 @@ export declare class DefaultUrlSerializer implements UrlSerializer {
     serialize(tree: UrlTree): string;
 }
 
+/** @deprecated */
+export declare type DeprecatedLoadChildren = string;
+
 export declare type DetachedRouteHandle = {};
 
 export declare type Event = RouterEvent | RouteConfigLoadStart | RouteConfigLoadEnd | ChildActivationStart | ChildActivationEnd | ActivationStart | ActivationEnd | Scroll;
@@ -145,7 +148,7 @@ export declare class GuardsCheckStart extends RouterEvent {
     toString(): string;
 }
 
-export declare type LoadChildren = string | LoadChildrenCallback;
+export declare type LoadChildren = LoadChildrenCallback | DeprecatedLoadChildren;
 
 export declare type LoadChildrenCallback = () => Type<any> | NgModuleFactory<any> | Observable<Type<any>> | Promise<NgModuleFactory<any> | Type<any> | any>;