create to_labeled_stacked_array helper function

[OriolAbril]

xarray has a to_stacked_array method, which is very handy. However, given the extra conventions and features in ArviZ, we could have an improved version of that, something like:

idata = az.load_arviz_data("centered_eight")

labeled_stack = idata.posterior.to_stacked_array("label", sample_dims=("chain", "draw"), name="posterior")
labels = [az.labels.BaseLabeller().make_label_flat(var_name, sel, isel) for var_name, sel, isel in az.sel_utils.xarray_sel_iter(idata.posterior, skip_dims={"chain", "draw"})]
labeled_stack = labeled_stack.reset_index("label").assign_coords(label=labels).set_xindex("variable").set_xindex("school")

were instead of a multiindex we use explicit indexes, use the labeller, and automatically set the name to the group name if available.

I think the function should have some similarities to extract with respect to the signature (i.e. accept idata or dataset, probably filtering vars too) but be an independent function.