Allow explicit support for `str` inputs to `NestedSeries.explode`, `NestSeriesAccessor.to_flat`, `NestSeriesAccessor.to_lists``

[dougbrn]

It's natural for all of these to invoke something like:

ns.explode("sub_column")

but we don't have the explicit string handler (to just store it in a list up front) so strange behavior can happen when trying to do this. Let's add the string handler. Extra motivated by the deprecation of get_flat_series, as this becomes more important as a performant replacement