A lazybox element can be single or multiple, which means it allows users to either select one, or multiple values.
Its selection_callback, which is executed each time a selection, or deselection occurs, takes different types of parameter depending of the type of lazybox:
unknown | null for single lazyboxesunknown[] for multiple lazyboxes
The goal of the refactoring is to make this callback always take an array as parameter.