I was thinking of bundling the following into this PR, but also happy if you want to merge this and I'll send another PR when the rest is ready:

• Add warning when saving jsonl file with no rows
• Add JSONL arrays loader
• Add JSONL arrays saver
• Add option to save JSONL as arrays
• Add extended warning which hints to use JSONL arrays format to preserve headers
• Add tests

We can sit on it for a bit while you're actively developing, see if we want to tweak the message or anything.

@saulpw, I think I'm ready for you to look at this now. I've made a few potentially controversial choices, so keen to hear your thoughts! A summary of my questionable choices follows:

• JSONL arrays get their own sheet type, inheriting from SequencSheet
• JSONL arrays get their own file extension (.jsonla)
• Loading as JSONL arrays automatically falls back to the default JSONL loader if the file doesn't include a JSONL arrays header
• I've split the json_save_arrays option in two: json_save_arrays and json_load_arrays. The first will always use the arrays format for .jsonl files, and the second will always attempt use the arrays format for .jsonl files, falling back to the default if the header is not detected.

From a user perspective, there is no change if they save as .jsonl and have the new options unset.

I haven't added any tests yet, so I'll look into that.

EDIT: Sorry, I think I understand now...

Perhaps I don't understand how testing works.
But it looks to me that the existing save-json.vd test doesn't actually save anything?
https://github.com/saulpw/visidata/blob/develop/tests/save-json.vd

Hmm... Testing has revealed a bug when saving as .jsonla.

I think we're good now.

Edit: Helps to commit the sample data...

Edit 2: Removed some vestigial code and force pushed

Thanks @saulpw, I've pushed some commits with the requested changes.