FIX: Make IsFirst and IsLast work as expected for PaginatedList (fixes #11465)

[kinglozzer]

Description

Adds a fix for #11465 in CMS 5 that doesn’t risk a BC-break.

Manual testing steps

Set up a paginated list (e.g. in your Page class):

public function getPaginatedList()
{
    $items = [
        ['Val' => 1],
        ['Val' => 2],
        ['Val' => 3],
        ['Val' => 4],
        ['Val' => 5],
        ['Val' => 6],
        ['Val' => 7],
        ['Val' => 8],
        ['Val' => 9],
        ['Val' => 10],
    ];
    $list = new ArrayList($items);
    $paginated = new PaginatedList($list);
    $paginated->setPageLength(3);
    $paginated->setCurrentPage(2);
    return $paginated;
}

In a template, iterate through that paginated list:

<ul>
    <% loop $PaginatedList %>
    <li>$FirstLast $Val</li>
    <% end_loop %>
</ul>

Verify that the output is:

    first 4
    5
    last 6

Issues

• IsLast doesn’t work when PaginatedList has more than one page #11465

Pull request checklist

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• This change is covered with tests (or tests aren't necessary for this change)
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• CI is green

[GuySartorelli]

LGTM for CMS 5.

Note this won't merge up straight to CMS 6 since the template engine is in a separate repository for 6 - and I personally think changing the count() behaviour of PaginatedList is a cleaner long-term solution anyway.

Won't merge right away in case there's still some concern about this behaviour - I'll merge by the end of the week if nobody raises any concerns (if I forget just @ me)