feat(mat-menu): Open nested menus should be delayed on hover.

### Feature Description

An option to delay the menu of nested items would be great.
This prevents the menus from blinking/flashing when you just hover/move the mouse quickly to another wanted item.

Solution: Add delay before opening the nested menu. Only a few milliseconds to prevent flashing menus when you just move the mouse around. The user can still click on a nested item to trigger the menu instantly. This will improve the UX. It's really annoying if the menus are flashing.

You can test my suggested behavior in other proven menus like in Windows 10. Just open a folder and open the context menu. You'll see that the nested menus don't open immediately. They are delayed. Same behavior in VSCode. A shorter delay than the Windows menus, but they are delayed. And I think we should implement this for the mat-menus too.

|mat-menu| vs-code |
|---|---|
|  |  |

https://material.angular.io/components/menu/examples

Summary:
- Delay nested menu on hover.
- Open nested menu instantly on click.
- Delay nested menu before close (hover leave).
- Implement it with fixed delay time or as an option property with default value (the time in ms. Not just boolean).

Btw: Another related issue which is about delay before closing: #24476
We could combine both issues to add delay before and after.

I think this issue is more a bug than a feature. An unwanted behavior / bad UX. But anyway, it's no candidate for votes. It something what should be considered by the Angular team.

Beginne mit der mat-menu-Implementierung und dem zugehörigen Problem zur Verzögerung beim Schließen #24476; vergleiche anschließend das gewünschte Verhalten mit den verknüpften Material-Menübeispielen. Als erledigt gilt, wenn verschachtelte Menüs nach einer kurzen Hover-Verzögerung geöffnet werden, sich beim Klicken sofort öffnen, vor dem Schließen eine Verzögerung besteht und eine feste oder konfigurierbare Standardverzögerung bereitgestellt wird.

Vom Indexierungsmodell aus dem Issue-Text verfasst.