When adding selections above/below with `skip_soft_wrap: true`, use
buffer column positions instead of pixel positions. This ensures
selections are placed at the same column offset in the buffer rather
than at the same visual position, which was incorrect for soft-wrapped
lines.

For example, selecting "how" in a wrapped line:

````
1. Very long line to
   show [how] a wrapped
   line would look
2. Very long line to
   show how a wrapped
   line would look
````

Now correctly adds a selection at the same buffer column in the next
line:

````
1. Very long line to
   show [how] a wrapped
   line would look
2. Very long line to
   show [how] a wrapped
   line would look
````

Closes #42137

Release Notes:

- Improved `editor: add selection above` and `editor: add selection
below` to select at the same text column when skipping soft-wrapped
lines, instead of the same visual position