Groupings

[RafDevX]

Please allow grouping objects. Containers are not flexible enough as objects cannot be in multiple containers at once (unless container A is fully contained within container B).

For example:

This is a very important feature for my use case and it might be a dealbreaker to not be able to place the same object in several different groups.

[alixander]

related https://github.com/terrastruct/d2/discussions/328