feat(FormField): signal forms support

### Feature Description

The first (experimental) version of Signal Forms will land in Angular 21.
This is awesome news, because with Signal Forms, forms finally become fully reactive, functional and composable.
I would like to be able to use Signal Forms with Angular Material.

For an outsider, it is impossible to see whether Angular Material support for Signal Forms:
1. Will just work out-of-the-box
2. Will require some small changes in Angular Material
3. Requires a huge amount of work in Angular Material

### Use Case

As a developer, I love the improvements that Signal Forms will bring to our code base.
I am really looking forward to using them in tandem with Angular Material.

Commencez par examiner Angular 21 Signal Forms et les composants form-field existants d’Angular Material. Déterminez si Signal Forms fonctionnent tels quels ou nécessitent des modifications, puis documentez les composants, les tests et le résultat de compatibilité concernés.

Rédigé par le modèle d'indexation à partir du texte de l'issue.