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?

从 issue 中链接的 Angular Material typography guide 开始，追踪 typography definition 如何表示 .mat-h5 和 .mat-h6 样式。在定位相关实现和测试之前，确定预期的自定义行为和 API。完成的标准是用户可以为自己的 guide 定义彼此独立的 H5 和 H6 typography styles。

由索引模型根据 Issue 内容生成。