[RFC]: add `ndarray/base/unflatten-shape`

[kgryte]

This is a complement to https://github.com/stdlib-js/stdlib/tree/develop/lib/node_modules/%40stdlib/ndarray/base/flatten-shape and is similar to https://docs.pytorch.org/docs/stable/generated/torch.unflatten.html, but only operates on shapes.

API signature:

function unflattenShape( shape, dim, sizes )

The implementation should validate that the product of sizes equals the size of shape[dim].