ENH: add broadcast_shapes

[lucascolley]

This is low priority, because you can just use https://numpy.org/doc/2.1/reference/generated/numpy.broadcast_shapes.html as the arguments are tuples. It would still be good to offer this for libraries which do not want to depend on NumPy.

[crusaderky]

np.broadcast_shapes won't work when the arguments have None (ndonnx) or math.nan (Dask) in them.

xp.broadcast_arrays(...)[0].shape works, but is very weak about the guarantee that the backend won't perform a deep-copy of the input data upon broadcast.

[crusaderky]

XREF data-apis/array-api#893