[Search] Match position indicator does not update when matches number changes

### Expected
If the note is edited while search mode is active, the position indicator (`2 of 5` thing) should update the current position digit, if matches number changes.

### Observed
Current position indicator never changes regardless of matches number:

https://user-images.githubusercontent.com/73365754/112330234-86e9a280-8cc0-11eb-895e-d97d49d43ac9.mov

### Reproduced
1. Have a note with multiple instances of `word` in it
2. Search for `word`
3. Move the current match position in a note with `>` button or with `Ctrl/Cmd+G`, so that it becomes greater than one (e.g. `5 of 5`)
4. Start removing the instances of `word` until one remains ~> the indicator will show `5 of 1`

### Where did you see the bug
For example in
- System Make: Parallels Desktop on MBP 16"
- OS: Windows
- OS version: 10 Home 20H2 build 19042.508
- Simplenote app version: 2.9.0-beta1 desktop

Reproduce the issue in the Simplenote desktop app by editing a note while search mode is active and reducing the number of matches. Trace the search position indicator update path; done means the current-position digit stays valid when matches change, including the reproduced `5 of 1` case.

Written by the indexing model from the issue text.