[list-box] Refactor API for configuring item checkmark

[web-padawan]

1. in vaadin-item we have this code:

:host([tabindex])::before {
  display: var(--_lumo-item-selected-icon-display, none);
  content: var(--lumo-icons-checkmark);
}

2. In vaadin-list-box we use it like this

:host {
  --_lumo-item-selected-icon-display: var(--_lumo-list-box-item-selected-icon-display, block);
}

3. In vaadin-context-menu, we override it like this:

:host {
  --_lumo-list-box-item-selected-icon-display: none;
}

4. However, in context-menu with nested menus, we reset it like this:

:host(.vaadin-menu-list-box) {
  --_lumo-list-box-item-selected-icon-display: block;
}

In future major releases, we need to come up with a better API.

[web-padawan]

Proper solution for this would require adding selectable to ItemMixin: #421 and then set it to true in case of vaadin-context-menu-item when using items property and false otherwise. I'll create a draft PR with a prototype.

[web-padawan]

This will be solved in base styles using --vaadin-item-checkmark-display. We can consider using it in Lumo too.