Add function to return multiple rows in map for withMapping

[dencarlsson]

Summary

This update allows map() to return multiple rows per item when exporting to CSV.
If the first element returned from map() is an array, it will now be treated as a list of sub-rows, which are iterated and written individually.
This function is available in laravel-excel.

Example use case

A typical scenario is when exporting data from a parent model with related items — for example, Users with their Cars.
Each car should produce its own CSV row, but it’s more convenient to reuse the existing logic and filtering on the User model rather than switch to the Car model and duplicate logic and/or filtering.