Set aria-setsize on grouped tabs to the number of tabs in the tab group. Set aria-posinset on grouped tabs to the 1-based index of the tab within the tab group. This allows some a11y tools to report to the user that a tab is, for example, tab "2 of 7" in a tab group.
The tab strip uses the `tablist` ARIA role. The ARIA spec and Firefox's a11y engine both forbid nesting `group` ARIA roles inside of `tablist`. As a result, a11y tools always read individual tabs as being tab "X of Y", where Y is the number of tabs in the tab strip and X is the index of the tab in the tab strip. This is good information, but it does not give the user a sense of where a tab is within a tab group.
Differential Revision: https://phabricator.services.mozilla.com/D245185
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