Sort value_counts result

Thoughts on adding a `sort_values(ascending=False)` to the end of `.value_counts()` (maybe especially if there is a single output partition)? I think that we used to do something like this historically.

Start by locating the implementation of `.value_counts()` and how it handles output partitions. Compare the current result with the proposed descending `sort_values` behavior, including the single-output-partition case; done means the intended sorting semantics are agreed and covered by tests.

Written by the indexing model from the issue text.