Insertion of an interlink via `[` deletes the preceding character

### Expected
Insertion of an interlink should not affect the previously created note content

### Observed
Adding an internal link via `[` will erase the preceding character:

https://user-images.githubusercontent.com/73365754/111192110-65523200-85c1-11eb-9a0a-7123285058ca.mov

This also happens to spaces in bulleted lists, which breaks the lists.

### Reproduced
1. Have at least two notes
2. Select any note and make sure the cursor is not at the 0 position of a line (i.e. have some content in the current line)
3. Insert an interlink by using `[` ~> The character on the left of `[` will be erased.

### Where did you see the bug
Video was recorded in Safari on Mac, but also reproducible for example in:

- System Make: Parallels Desktop on MBP 16"
- OS: Windows
- OS version: 10 Home 20H2 build 19042.508
- Simplenote app version: 2.8.0-beta2 desktop

Reproduce the issue in the Simplenote desktop app using the numbered steps, including a bulleted list and a cursor away from line position 0. Trace the interlink insertion path triggered by `[`, then verify that the preceding character or list space remains unchanged across the reported environments.

Written by the indexing model from the issue text.