feat(theming): Mixins for reading component style tokens

[martinboue]

Feature Description

Mixins to get values from a component theme that takes a token as parameter, for example:

p {
   color: @include mat.button-get(text-label-text-color);
}

it would generate:

p {
   color: var(--mdc-outlined-button-label-text-color, var(--mat-sys-primary)); // <== Actual value used for text-button
}

Ideally, we would have as many mixins as there are components (+core) and we could use the same tokens as the mat.<component>-overrides mixins.

The mat.<component>-get mixin name is a proposal, open to suggestions.

Use Case

I have a custom component and I want to apply the same style as a material component. I also want to make sure that they are always in sync when I change my theme.

For example, I want to have a text <p> with the same font color as my mat-button next to it.

For now, I need to rely on private CSS variables to do that, which is not ideal for maintainability.