Add index on `item_version._item`

[simonw]

Looking up all versions of a specific item is a very common operation.

[simonw]

But when should I add this index? Ideally I would add it when the table is first created, but doing that concisely is blocked on:

• https://github.com/simonw/sqlite-utils/issues/349

So I guess I add the index at the very end of the script instead, at least for the moment.