Accordion icons and custom slots

Accordion supports custom trigger icons and end content. Avatar displays initials in medium weight.

New features

• Added icon to Accordion.Root to replace the default chevron on each trigger.
• Added endSlot to Accordion.Root to replace the icon with non-interactive content, or hide it with null.
• Added useAccordionItem to read an item's active and disabled state.

Improvements

• Changed Avatar initials from semibold to medium font weight.