Count Nested: Sort output columns of `by` behavior alphabetically for nested-dask meta

[dougbrn]

Feature request
When by is set for count_nested, the output columns are ordered as per value_counts default behavior. This ordering is value-based and thus unpredictable for dask. We should have by output columns be sorted alphabetically to create a meta-friendly output for Nested-Dask. The PR should just involve implementing the behavior like this:

else:
        # this may be able to be sped up using tolists() as well
        counts = df[nested].apply(lambda x: x[by].value_counts(sort=False))
        counts = counts.rename(columns={colname: f"n_{nested}_{colname}" for colname in counts.columns})
        counts = counts.reindex(sorted(counts.columns), axis=1)

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