feat: get item index in itemLabel on Repeater

[pintend]

passes the index to getItemLabel() so we can do something like this

Repeater::make()
    ->itemLabel(function (int $index) {
        return match($index) {
            1 => 'Primary',
            default => 'Backup',
        };
    })

[People-Sea]

Please rebase onto 4.x

[People-Sea]

Should this be $loop->index instead? Since the injected parameter is named $index, it should start at 0 to match convention.

[danharrin]

Please reopen on 4.x

[pintend]

Replaced by #19430