`GridModel` should support auto-sizing the tree column on expand state change

This is something fairly common to do in apps and makes for a nice UX in tree grids with several levels, especially with long child-dimension values. We could add to the `GridAutosizeOptions`, the reaction to do so should be easy to wire up since we have observable expand state already. Need to consider how this would work with the existing `includeCollapsedChildren` option.