Remove aria-haspopup=dialog and add role=button

- aria-haspopup=dialog
Using NVDA as an example: nvaccess/nvda#8235

- role=button
While the default summary role (disclosure triangle) could be
appropriate elsewhere, dialog's trigger element should be announced as
a button.

index.js

@@ -196,7 +196,7 @@ class DetailsDialogElement extends HTMLElement {
 
     const summary = details.querySelector('summary')
     if (summary) {
-      summary.setAttribute('aria-haspopup', 'dialog')
+      summary.setAttribute('role', 'button')
       summary.addEventListener('click', onSummaryClick, {capture: true})
     }