build: enable bundle_dts for router package (#28833)

This PR also changes the name of `EmptyOutletComponent` to `ɵEmptyOutletComponent`. This is because `ngcc` requires the node to retain the original name while dts bundler will rename the node is it's only exported using the aliases.

Example typings files:
```ts
declare class EmptyOutletComponent {
}
export {EmptyOutletComponent as ɵEmptyOutletComponent}
```

will be emitted as
```ts
export declare class ɵEmptyOutletComponent {
}
```

PR Close #28833

packages/router/src/components/empty_outlet.ts

@@ -18,5 +18,7 @@ import {Component} from '@angular/core';
  * to this `EmptyOutletComponent`.
  */
 @Component({template: `<router-outlet></router-outlet>`})
-export class EmptyOutletComponent {
-}
+export class ɵEmptyOutletComponent {
+}
+
+export {ɵEmptyOutletComponent as EmptyOutletComponent};