Returns the index of the current page of results.
currentPage is a zero-based index of the current page of results, meaning the 1st page is 0, the 2nd page is 1, and so on.
If no results match the query, currentPage is undefined.
The page size is defined by the limit() function and can be retrieved using the pageSize property. You can use the next() and prev() functions returned by plans to navigate the pages of a query result.