fix(VListItem): only attach onClick when item is clickable

[sridhar-3009]

Closes #22854

Problem

NVDA and other screen readers detect VListItem as "clickable" even when the item has no interactive functionality. This happens because onClick={ onClick } was always attached unconditionally to every VListItem, causing screen readers to announce all list items as interactive/clickable.

Fix

Made the onClick listener conditional:

// Before
onClick={ onClick }

// After  
onClick={ (isClickable.value || attrs.onClick) ? onClick : undefined }

The listener is now only attached when:

1. The item is actually clickable (isClickable.value — has a link, is selectable, etc.)
2. OR a parent component has explicitly passed an @click handler (attrs.onClick)

The emit('click', e) still fires in both cases since it's inside the onClick function.