mat-menu-item width

#### Feature Description

I would like to set the width of the mat-menu-item to the same with as the button wich opens the menu.

#### Use Case

I have a menu and a larger button open button. It would look so much better if the mat-menu-item has the same width as the button. Actually there is no simple option.

定位 mat-menu-item 和 menu 的实现及其现有测试，然后阅读菜单宽度和触发器尺寸的处理方式。完成标准是：某个受支持的选项能够使菜单项宽度与打开菜单的按钮宽度一致，并且测试覆盖所请求的行为。

由索引模型根据 Issue 内容生成。