fix: prompt glitch when resizing with cursor in a multiline command

[blindFS]

Fixes a bug overlooked in #841 (forgot that the before_cursor string could have newlines in it too).

Before:

https://github.com/user-attachments/assets/cf79d8db-24c8-45f0-9a6d-f0092daaa7eb

After: stays in the same row.