feat(ui5-menu): menu item groups with checkable menu items (#10028)

This PR introduces **MenuItemGroup** component that can hold regular **MenuItem** components. The **MenuItemGroup** has a property named `itemCheckMode` which can have values among `None` (default), `Single` and `Multiple`. **MenuItemGroup** can be slotted in a **Menu** or **MenuItem** default slot as any other regular **MenuItem**. Nesting of **MenuItemGroup** components is not allowed, but any **Menu** or **MenuItem** can contain more than one **MenuItemGroup** components with different `itemCheckMode` settings.

When `itemCheckMode` is:

* `None`, the **Menu** acts exactly like until now. 
* `Single` means that zero or one **MenuItems** can be checked at a time.
* `Multiple` means that zero or many **MenuItems** can be checked at a time.

There is also new property `checked` introduced in **MenuItem**. By setting it the item is marked as **checked** and this is visualized as checkmark at the end of the item. This property is taken into account only when the corresponding item is a member of MenuItemGroup with `Single` or `Multiple` value of `itemCheckMode`.

![image](https://github.com/user-attachments/assets/13c409bc-bd97-4815-8c1b-b9ffc81eb8b2)

It is recommended to place separators before and after each **MenuItemGroup**, but this is not mandatory and depends on the application developers.

Author: NHristov-sap

packages/fiori/src/UserMenuItem.ts

@@ -1,5 +1,5 @@
 import { customElement, slot } from "@ui5/webcomponents-base/dist/decorators.js";
-import MenuItem from "@ui5/webcomponents/dist/MenuItem.js";
+import MenuItem, { isInstanceOfMenuItem } from "@ui5/webcomponents/dist/MenuItem.js";
 
 import UserMenuItemTemplate from "./UserMenuItemTemplate.js";
 
@@ -45,7 +45,7 @@ class UserMenuItem extends MenuItem {
 	declare items: Array<UserMenuItem>;
 
 	get _menuItems() {
-		return this.items.filter(item => !item.isSeparator);
+		return this.items.filter(isInstanceOfMenuItem);
 	}
 }