fix: ak.argcombinations should allow negative axis

[pfackeldey]

@jpivarski looks good to me! I wonder: do you know why this was forbidden in the first place?

[jpivarski]

I think it's just always been an oversight.

• ak.argcombinations and ak.combinations were ported over from the Awkward 1 C++ in #1307, and ak.argcombinations has the axis >= 0 restriction there, while ak.combinations doesn't. ak.argcombinations was written in terms of ak.combinations and ak.localindex, as it is now.
• ak.argchoose and ak.choose (the old names) were orinally written in Awkward 1 in #160, and ak.argchoose has the axis >= 0 restriction there, while ak.choose doesn't. ak.argchoose was written in terms of ak.combinations and localindex, as it is now.
• argchoose and choose didn't have an axis argument in Awkward 0. (choose was written in terms of argchoose, by applying the argchoose result as an integer slice, so that's different.)

When I was adding axis arguments to everything in the Awkward 0 → 1 transition, I must have thought, "There's no way this is well-defined (or implementable) if it has an axis argument" for argchoose but then had a different thought in choose. I probably never looked at them side by side like this. It is not the case that they originally both had the restriction and it was lifted from choose. So, it's not a maintenance inconsistency; it's a development inconsistency.