Add FileAttachment().jsonl() support

[gisle]

I would like to see support for new-line separated JSON, either by adding another method to FileAttachment() or by adding some option to the json() method.

For instance the Python Panadas library has implemented this with a lines = True option to their read_json() function.

My current ugly workaround is to add this to my Observable notebook:

data = FileAttachment("data.jsonl").text()
  .then(content => content.split("\n").filter(line => line).map(line => JSON.parse(line)))