Implement Dataset from JSON and JSON Lines
Thanks @lhoestq. I was trying to follow @thomwolf suggestion about integrating that script but as `from_json` method...
> Note that I don't think this is necessary a breaking change, we can still keep the old scripts around

Do you think there is a better way of doing it?

I was trying to implement more or less the same logic as in the script, but I confess I assumed the target was in-memory only... Basically, I was trying to reimplement `Json(datasets.ArrowBasedBuilder)._generate_tables`, and no writing to arrow file (I assumed only in-memory usage). I started with the first "else" clause... 

I was planning to remove my `_cast_table_to_info_features` and use `paj.read_json(parse_options=...)` instead (like in the script). @lhoestq I am wondering why `keep_in_memory` has no effect for JSON... What's the issue exactly ? Apparently it's correctly passed to as_dataset so I don't find the issue Nevermind @lhoestq, I found where the problem was in my code... I push! <s>merging master into this branch should fix the CI issue :)</s>

Oops I didn't refresh the page sorry ^^'

Looks all good ! Good job ! I think we can merge after the last changes regarding the error message and the docstring above :) @lhoestq Done! And I have also added some tests for the `field` parameter. Let me add some more tests for dict of lists JSON file, please. @lhoestq done! ;) We can merge. Additional work will be done in another PR. ;)
Implement `Dataset.from_jsonl`.