Angular Material Typography: No ability to customize H5 and H6 levels in typography definition

### Feature Description

Quote from https://material.angular.io/guide/typography:
_The .mat-h5 and .mat-h6 styles don't directly correspond to a specific Material Design typography level. The .mat-h5 style uses the body-1 level with the font-size scaled down by 0.83. The .mat-h6 style uses the body-1 level with the font-size scaled down by 0.67._

### Use Case

We have a specific typography guide to implement but have no defined styles for h5 or h6, do to it not corresponding directly to a typography level.

Are there ways to do this?

Start with the Angular Material typography guide linked in the issue and trace how the typography definition represents the .mat-h5 and .mat-h6 styles. Determine the intended customization behavior and API before locating the relevant implementation and tests. Done means users can define independent H5 and H6 typography styles for their own guide.

Written by the indexing model from the issue text.