Develop design for more structured data

[zastrowm]

There are specific controls that take more advanced data structures, such as trees or lists, and we need to design how we expect those controls/structures will be edited and rendered in a way that provides flexibility but doesn't include make it overly complex for non-coding users.

Longer term, we should consider whether templates would solve enough use cases (see https://sheetui.com/ for inspiration) or whether we can incorporate a "mini-instance" of the designer for each sub-item.