A11y: Use <button> as default wrapper for popover menu items

[bettysteger]

Currently, popover menu items in PopoverDesktop are rendered as <div> elements by default. This requires consumers to opt in to semantic markup by passing wrapperTag: 'button' via PopoverItemRenderParamsMap, which is easy to miss.

Why this matters

A <div> has no implicit role, is not keyboard-focusable by default, and gives screen readers no actionable context. A <button> element gives you all of this for free:

• role="button" out of the box
• Focusable and activatable via Enter/Space without extra ARIA
• Native disabled attribute support, which automatically sets aria-disabled in browsers and prevents activation
• Correctly announced by screen readers as an interactive element

Proposed change

Change the default wrapperTag for PopoverItemType.Default from 'div' to 'button'. The existing wrapperTag option in PopoverItemRenderParamsMap can remain for consumers who
have a reason to override it.

Current workaround

new PopoverDesktop(params, {                                                                                                                                                 
  [PopoverItemType.Default]: { wrapperTag: 'button' }     
});            

This works, but it shouldn't be the consumer's responsibility to enable baseline accessibility.