Default styles for VirtualList items

[rolfsmeds]

Describe your motivation

As a followup to vaadin/platform#6838, it would be good to have some opt-in default visualizations for the focused and selected state of VirtualList items. Perhaps these could even be combined with some kind of default styles for the item wrappers?

Describe the solution you'd like

Some general item wrapper styles that come to mind:

Raised card with rounded borders and shadow

• Focus indicated with ring
• Selection indicated with background color or emphasized outline

Flat, with separators between items (somewhat like Grid rows)

- Focus indicated with ring (0 radius) - Selection indicated with background color

ListBox style

- Focus indicated with ring for keyboard, hover style for pointer - Selection indicated with checkmark

Presumably these would be implemented as theme variants on the VirtualList component.

Describe alternatives you've considered

No response

Additional context

No response