feat: add indeterminate state to parent rows when a child is selected

[lukasnys]

Why?

It's a very common pattern when working with nested checkboxes, that - when selecting a child checkbox - the parent checkbox gets an indeterminate state

What?

Add this indeterminate behaviour

[lukasnys]

I need to change the indeterminate state so all parents (even ones further than 1 level) get isGroupIndeterminate: true when a child is selected.