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.)

Beginne damit, src/material/form-field/form-field.html an der verlinkten Stelle für das Rendern von Hints zu lesen und zu prüfen, wie MatHint und MatError derzeit koordiniert werden. Vergleiche den vorgeschlagenen CSS-Persistenzansatz mit der alternativen persistHint-Eingabe und identifiziere dann die erforderliche Designentscheidung für die Anzeige beider. Als erledigt gilt die Aufgabe, wenn das gewählte Verhalten ohne das derzeitige Entfernen aus dem DOM implementiert und durch die relevanten form-field-Tests abgedeckt ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.