Helper class for page and page item counting
const helper = new PaginationHelper(['a','b','c','d','e','f','g','h'], 3);
// retrieve number of pages (1-based)
helper.pageCount() // 3
// retrieve number of items
helper.itemCount() // 8
// retrieve number of items on this (as provided by the argument) page (0-based)
helper.pageItemCount(2) // 2
// retrieve number of the page the specified item will be displayed on (item number and page number are 0-based)
helper.pageIndex(7) // 2