fix: export nan variants

[agoose77]

We have high-level ak.XXX reducers that include nan-aware variants. We should export those!

[codecov]

Codecov Report

Merging #2758 (9ddc907) into main (2748c0c) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/operations/ak_argmax.py	87.50% <100.00%> (ø)
src/awkward/operations/ak_argmin.py	87.50% <100.00%> (ø)
src/awkward/operations/ak_max.py	87.50% <100.00%> (ø)
src/awkward/operations/ak_mean.py	87.23% <100.00%> (ø)
src/awkward/operations/ak_min.py	87.50% <100.00%> (ø)
src/awkward/operations/ak_prod.py	87.50% <100.00%> (ø)
src/awkward/operations/ak_std.py	83.72% <100.00%> (ø)
src/awkward/operations/ak_sum.py	87.50% <100.00%> (ø)
src/awkward/operations/ak_var.py	82.00% <100.00%> (ø)

[jpivarski]

Oops; that was added and tested in awkward._v2 (#1325 (comment)), but must have gotten lost when the submodule got promoted.

I'll merge this now.