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
}
```

まず、要求されている SCSS breakpoint 変数を、issue で言及されている CDK Layout API および Angular Material の theming サーフェスと比較します。Angular Material が、各プロジェクトで再定義せずに SCSS media query で使用できる、文書化された breakpoint 値を公開すれば完了です。

索引モデルが issue の本文から書いたものです。