datatree: Collapsible items in groups DataTree

[owenlittlejohns]

What is your issue?

Originally posted by @agrouaze in xarray-contrib/datatree#145
Attempted implementation in this PR

Having collapsible items (like xarray.Datasets) in groups repr_html_ would help to have user friendly overview of Python objects.
I am wondering whether this feature is already available or not.

Clarification: The ask here is to make individual groups collapsible in the HTML rendering, so it is possible to see all the child groups, without having to see the full contents of a group.

[owenlittlejohns]

When migrating this issue, we discussed that this work may need to (or wait for) be part of a larger rewrite of the HTML rendering code.