feat(router): Allow loadChildren to return a Route array

[atscott]

This commit expands the LoadChildrenCallback to accept returning Routes
in addition to the existing NgModule type. In addition, it adds a
check to ensure these loaded routes all use standalone components.
The components must be standalone because if they were not,
we would not have the required NgModule which the component is declared in.

Existing API:

{path: 'lazy/route', loadChildren: import('./lazy').then(m => m.LazyModule)}

@NgModule({
  imports: [
    ExtraCmpModule,
    RouterModule.forChild([
      {path: 'extra/route', component: ExtraCmp},
    ]),
  ],
})
export class LazyModule {}

The new API for lazy loading route configs with standalone components
(no NgModule) is to expand loadChildren to allow returning simply a Routes array.

// parent.ts
{
  path: 'parent',
  loadChildren: () => import('./children').then(m => m.ROUTES),
}

// children.ts
export const ROUTES: Route[] = [
  {path: 'child', component: ChildCmp},
];

Note that this includes minimal documentation updates. We need to
include a holistic update to the documentation for standalone components
in the future that includes this feature.

[AndrewKushnir]

👍

[jessicajaniuk]

reviewed-for: public-api

[alxhub]

Reviewed-for: public-api, fw-core, fw-router

[AndrewKushnir]

Looks great 👍

[AndrewKushnir]

Reviewed-for: public-api

[atscott]

presubmit

merge assistance: requires an update to the g3 patch: http://cl/443671599

[atscott]

This PR was merged into the repository by commit 4962a4a.

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