feat: add a wildcard for _json columns

[ytjohn]

This allows _json to accept a wildcard for when there are many JSON columns that the user wants to convert. I hope this is useful. I've tested it on our datasette and haven't ran into any issues. I imagine on a large set of results, there could be some performance issues, but it will probably be negligible for most use cases.

On a side note, I ran into an issue where I had to upgrade black on my system beyond the pinned version in setup.py. Here is the upstream issue <psf/black#2964 . I didn't include this in the PR yet since I didn't look into the issue too far, but I can if you would like.

[simonw]

Interesting feature idea.

It's a weird edge-case, but it's the kind of thing I worry about: what if the user has a column called *? This is technically allowed in SQLite, even though it's vastly unlikely.

On that basis, I think ?_json_all=1 would be a better design here than ?_json=* - since it removes any possible ambiguity.