fix: support categorical counts in ak.unflatten

[agoose77]

Fully closes #2071 by generalising support for indexed types to any indexed layout type (not just non-categoricals).

[codecov]

Codecov Report

Merging #2099 (6cd9cc3) into main (4db99b6) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/_connect/numpy.py	68.96% <100.00%> (+0.43%)	⬆️
src/awkward/_util.py	82.07% <100.00%> (+0.25%)	⬆️
src/awkward/behaviors/string.py	76.66% <100.00%> (+0.63%)	⬆️
src/awkward/highlevel.py	76.21% <100.00%> (ø)
src/awkward/operations/ak_to_layout.py	88.37% <100.00%> (-0.76%)	⬇️
src/awkward/operations/ak_unflatten.py	95.45% <100.00%> (+0.14%)	⬆️
src/awkward/operations/ak_with_field.py	100.00% <100.00%> (ø)

[jpivarski]

That makes sense! to_packed wouldn't project out categoricals—that would be undesirable in most applications of packing. But here we want to do it.

This fixes it; merge whenever you're ready.