Support JSONL and incremental importing

[PeterJCLaw]

Currently the memory usage of devdata during an import can be fairly large -- it scales with the size of the export being imported. This can result in an import failing part way through due to the process being OOMKilled. While ideally devdata would only have smallish data to work with, that isn't always practical.

It would be great if devdata supported exporting to JSONL (i.e: newline separated JSON) as this can be parsed incrementally and could thus should be able to imported incrementally too.

[PeterJCLaw]

Had a quick look at this; it's a bit more complicated than might be hoped due to the way that we work with the tree of exported files -- we'd need to indicate or detect the format in several places, meaning there's potentially a bunch of passing through which might need to happen if we don't want to just rely on e.g: file extensions (which might be a reasonable way to go if we wanted to).