Checklist is inserted at wrong position if line starts from a bullet list marker

### Expected
`Insert checklist` should insert the checkbox at the beginning of the line (this is so in iOS, Android, macOS). E.g. have:

`+Plus` ~> user clicks `Insert checklist` ~> result is `☐ +Plus`

### Observed
`+Plus` ~> user clicks `Insert checklist` ~> result is `+- [ ] Plus`

https://user-images.githubusercontent.com/73365754/111340048-efad9b00-8680-11eb-8a8b-f3f163449775.mov

### Reproduced
1. Create a note that contains a line that starts with `-`, `+` or `*`. E.g. `*Hello*`
2. Select this line and insert checklist ~> the result will be `*- [ ] Hello*`

### Where did you see the bug
Video recorded on macOS but the same for example on:
- System Model: 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 from the listed steps in the Simplenote desktop app, then trace the Insert checklist command and its handling of lines beginning with -, +, or *. Done means the checkbox is inserted before the marker while preserving the existing text, with regression coverage for all three markers.

Written by the indexing model from the issue text.