Set child nodes via DataTree.children

What is your issue?

I realised that if you want to add new children to a DataTree object, then while this works

dt[name] = new_child

and you can examine the children using

dt.children # returns {'name': child1, ...}

then actually this does not currently work

dt.children[name] = new_child

It would be nice to make this work, as I think setting with .children makes the intent much clearer.

It doesn't work because .children is a property that returns a dict - the only way you can update the .children is by replacing it entirely using the property setter, i.e.

dt.children = {'name': new_child}

This problem is analogous to how Dataset has ds.coords, which returns a proxy mapping object which has a __setitem__ method that allow modification in-place.

sort of related to #9204 , and came up whilst doing #9297

cc @shoyer

Start by locating the DataTree.children property and comparing it with the Dataset.coords proxy mapping mentioned in the issue. Verify the existing dt[name] = new_child behavior, then confirm that dt.children[name] = new_child updates the tree in place and add coverage for that usage.