Transition containers to use Foundry's suggested method for handling them
Foundry's suggested method generally is keeping the contained items as parallel items that are hidden.
This although has its problems, solves a whole load of problems with contained items, though it also introduces new ones (namely how to handle weight of items in containers that reduce that weight).
There's also some challenges in how to represent containers like these in compendiums as the contents will be very difficult to grok among the extracted content as there's going to be large quantity of duplicates (probably should add support for compendium UUID links that all point to the same item that is not treated as actual contained item).
Other problems exist with copying containers as each layer of containment needs to be copied in separate steps due to item IDs not staying the same.
Optimally we'd also introduce helper function to retrieve all contained items. Container tree is easy to build from there.