Support for unstack

[AzeezIsh]

https://data-apis.org/array-api/latest/API_specification/generated/array_api.unstack.html

I would appreciate if you could update this with an unstack command!

[asmeurer]

2023.12 support is tracked at #25. Right now we are blocked on test suite support.

Perhaps once #30 is completed we can start adding 2023.12 functions piecemeal, since they would be disabled by default. unstack should match NumPy so there's little risk in adding it even without test suite support.