Checklist is inserted unevenly

It was reported in 3974770-zen that the "Insert Checklist" function causes list to have uneven spacing.

### Expected
`Insert Checklist` should only insert checkboxes at the beginning of the line and maintain uniform spacing.

### Observed
When you click `Insert Checklist`, it results in the list having uneven spacing.

### Reproduced

Make|Model|macOS Version|App Version
-|-|-|-
MacBook Pro|16-inch, 2019|Big Sur|2.11

Start by locating the macOS editor entry point for the “Insert Checklist” function and reproduce the uneven spacing shown in the attached screen capture. Verify that checklist markers are inserted only at the beginning of each line and that spacing remains uniform when the behavior is fixed.

Written by the indexing model from the issue text.