Support negative step size in indexing

[tomwhite]

From @dcherian in #636 (comment)

https://github.com/pydata/xarray/blob/99ee8c6ca54057a9b994d7685f36236f2d5a69d9/xarray/core/indexing.py#L1084 and friends. We rewrite the query to normal slice with +ve stride, then reverse in-memory after read :)

This is one of Xarray's magic tricks that very few people know about. We guarantee consistent indexing API over any array for the most part.

In pydata/xarray#8667, I suggested reusing this machinery for cubed.

EDIT: actually my comment is here: pydata/xarray#8834 (comment)

[tomwhite]

Note that when this is implemented we should include array_api_tests/test_array_object.py::test_getitem in the array api tests agains as it was added to the skip list in #636.