Previously, setting listsAsDuplicateKeys or listToValue would make it so
merging these treat all values as lists, by coercing non-lists via
lib.singleton. Some programs (such as gamemode; see #345121), allow some
values to be repeated but not others, which can lead to unexpected
behavior when non-list values are merged like this rather than throwing
an error.
This now makes that behavior opt-in via the mergeAsList option. Setting
mergeAsList (to either true or false) without setting either
listsAsDuplicateKeys or listToValue is an error, since lists are
meaningless in this case.