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.

Locate the mat-menu-item and menu implementation and its existing tests, then read how menu width and trigger sizing are handled. Done means a supported option makes the menu item width match the button that opens the menu, with tests covering the requested behavior.

Written by the indexing model from the issue text.