feat(mat-form-field): Option to allow MatHint to persist alongside MatError

### Feature Description

On every Angular Material project I've worked on, our accessibility team has flagged that the `mat-hint` disappears when `mat-error` appears. To be accessible, it is required that the `mat-hint` continues to persist somehow. Currently, the `mat-hint` element is fully removed from the DOM: https://github.com/angular/components/blob/master/src/material/form-field/form-field.html#L83

This has been brought up on SO several times:
- https://stackoverflow.com/questions/60185384/keep-the-displaying-of-both-error-and-hint-if-error
- https://stackoverflow.com/questions/47166694/angular-material-input-with-hint-text-and-error-text-at-the-same-time

To compensate, on each project we:

1. Write a wrapper component around `MatLabel`, `MatHint`, and `MatError`, as well as `MatInput`, `MatSelect`, `MatCheckbox`, etc, duplicating all of their inputs, or
2. Write a wrapper component around `MatLabel`, `MatHint`, and `MatError`, using content projection for the form controls, which breaks the synchronization between all the elements.

Either way, we spend a lot of time on ugly hacks with which we are always ultimately unsatisfied.

I propose we hide `mat-hint` with CSS (e.g. `display: none`) instead of fully removing it from the DOM. This will allow users to easily continue to persist `mat-hint` as they see fit, simply by overriding the CSS.

(Alternatively, or in addition, we could add e.g. `@Input('persistHint')` to `mat-form-field`, but this would require more of a reworking of `mat-form-field`'s logic, and would also require making some design decisions about how `mat-hint` should look when displayed alongside errors, for which the Material guidelines don't give instruction AFAIK.)

Inizia leggendo src/material/form-field/form-field.html nel punto collegato relativo al rendering degli hints e verifica come MatHint e MatError vengono attualmente coordinati. Confronta l’approccio proposto di persistenza tramite CSS con l’input alternativo persistHint, quindi individua la decisione progettuale necessaria per visualizzarli entrambi. Il lavoro è completato quando il comportamento scelto è implementato senza l’attuale rimozione dal DOM ed è coperto dai test pertinenti di form-field.

Scritto dal modello di indicizzazione a partire dal testo della issue.