Bug 1938624 part 2: Add a new SEARCHBOX Gecko a11y role. r=eeejay

ARIA roles which don't map to a specific Gecko role tend to lead to complexity and obscure edge cases.
This just adds the new role; it'll be mapped in subsequent patches.

Differential Revision: https://phabricator.services.mozilla.com/D233229

accessible/interfaces/nsIAccessibleRole.idl

@@ -813,4 +813,9 @@ interface nsIAccessibleRole : nsISupports
    * elements in an HTML table element.
    */
   const unsigned long ROLE_ROWGROUP = 139;
+
+  /**
+   * A type of textbox intended for specifying search criteria.
+   */
+  const unsigned long ROLE_SEARCHBOX = 140;
 };