feat: add `take_long_axis` to specifiation

[kgryte]

This PR

• resolves https://github.com/data-apis/array-api/issues/808 by adding support for selecting elements at one-dimensional indices along a specified axis.
• makes the axis kwarg keyword-only, which differs from the OP (see https://github.com/data-apis/array-api/issues/808). This is for consistency with axis kwargs elsewhere in the specification. The proposal seemed to follow PyTorch in allowing both positional and kwarg, which could be considered allowed behavior, but positional usage won't be portable.
• leaves out-of-bounds behavior unspecified.