Convert SQLite JSON columns to JSONB

[brandur]

Alright, so this one's been some cleanup that I've been intending for a
while. Previously, sqlc made using JSONB in SQLite completely unusable
because it sent back the binary format which you're explicitly not
supposed to parse yourself. I ended up fixing this like a year ago in
[1], but getting a sqlc release out the door took so long that I ended
up forgetting about it.

Here, modify our SQLite definitions so that JSON becomes JSONB, and add
migration to convert existing installation values to the same. Luckily,
this doesn't require a table rewrite because both JSON and JSONB are
SQLite BLOB types.

I didn't want to add a migration version just for SQLite, so to keep
everything in sync I also added a version 7 for Postgres that's just a
no-op with a comment.

[1] sqlc-dev/sqlc#3968

[brandur]

@bgentry Thoughts on this? I'm kinda wondering if we should try to bundle it up with #1115 (database cleanup) so we don't waste any migration versions. 007 would still be optional for Postgres since the cleanup wouldn't be strictly necessary.