This PR adds the `correctly_deleted_chars` field and updates `kept_rate`
to account for it, not just inserted chars.

It also adds `recall_rate` to measure coverage of reference
insertions/deletions.

Finally, it renames "final" to "reference" and "prediction" to
"candidate".

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable


Release Notes:

- N/A