Return scalar when accessing zero dimensional array

[brokkoli71]

• fixes Inconsistent () indexing/size behavior from v2->v3 #2713

0-dimensional arrays are now returning a scalar. Therefore, the return type of __getitem__ changed
to NDArrayOrScalarLike. This change is to make the behavior of 0-dimensional arrays consistent with
numpy scalars.

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.rst
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[brokkoli71]

Do we want to return scalars when accessing zero-dimensional arrays? This would mean that the return value of __getitem__ is no longer NDArrayLike and may make typing more difficult

[dstansby]

To copy what numpy does (see #2713 (comment)), I think we do want to return a scalar.

[brokkoli71]

@d-v-b what do you think about NDArrayLikeOrScalar return type for __getitem__? (better ideas for the name of NDArrayLikeOrScalar are also welcome)