group_split(): add a '.groups_to_names=T' type argument

[barnabasharris]

When splitting by categorical (or indeed integer) groups it might be useful to transfer the grouped values to the names of the resulting list object, to assist with functions later in the pipeline.

[etiennebacher]

I think this was already addressed in #4223 (and rejected)

[DavisVaughan]

Yea names on the list are not the right place for this information as it only works decently for single variable cases, and even then it isn't perfect.

I think the results of tidyr::nest(.by = ) are much more appropriate for this