Allow `doorstop import` to add new items.

Currently, only rows with UIDs will be processed.

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TODO:

- [x] refactor "get next number" code into `Document.next` using dependency injection
- [x] on export, add new rows with "(auto)" as the UID
- [x] update `doorstop.core.importer` to accept empty rows (with at least `text`)
- [x] on empty rows, create a new item (assigned the next UID)