feat(core): Expose SCSS breakpoints

### Feature Description

As @angular/flex-layout become deprecated and Angular team does not intend to provide an alternative, we are forced to handle responsive layout by our own.

Writing CSS is ok, but to stick with Material design guidance we must use specific breakpoints. It would be great if @angular/material could expose these breakpoints like it does for palettes and theming. Otherwise we have to declare them ourself for every projects.

### Use Case

would simplify scss writting and add opinions about breakpoints (like CDK Layout API)

```scss
@use '@angular/material' as mat;

@media #{$mat.gt-xs} {
// insert your styles
}
```

Inizia confrontando le variabili di breakpoint SCSS richieste con la CDK Layout API e la superficie di theming di Angular Material menzionate nell’issue. Il lavoro è completato quando Angular Material espone valori di breakpoint documentati che possono essere utilizzati nelle media query SCSS senza ridefinirli in ogni progetto.

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