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

packages/core/src/linker/ng_module_factory_loader.ts

@@ -17,6 +17,8 @@ import {NgModuleFactory} from './ng_module_factory';
  * Used to load ng module factories.
  *
  * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated, and `NgModuleFactoryLoader` is
+ * part of its implementation. See `LoadChildren` for more details.
  */
 export abstract class NgModuleFactoryLoader {
   abstract load(path: string): Promise<NgModuleFactory<any>>;

packages/core/src/linker/system_js_ng_module_factory_loader.ts

@@ -24,6 +24,8 @@ declare var System: any;
  * token.
  *
  * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated, and `SystemJsNgModuleLoaderConfig`
+ * is part of its implementation. See `LoadChildren` for more details.
  */
 export abstract class SystemJsNgModuleLoaderConfig {
   /**
@@ -47,6 +49,8 @@ const DEFAULT_CONFIG: SystemJsNgModuleLoaderConfig = {
 /**
  * NgModuleFactoryLoader that uses SystemJS to load NgModuleFactory
  * @publicApi
+ * @deprecated the `string` form of `loadChildren` is deprecated, and `SystemJsNgModuleLoader` is
+ * part of its implementation. See `LoadChildren` for more details.
  */
 @Injectable()
 export class SystemJsNgModuleLoader implements NgModuleFactoryLoader {