Fixes an issue where `editor::AddSelectionAbove` and
`editor::AddSelectionBelow`, when set with `skip_soft_wrap: true` ,
would lose track of the original cursor column when passing through
short lines, as `editor::Editor.add_selection` was not using the oldest
selection's column range when adding a new selection.

These changes update the `editor::Editor.add_selection` method so as to
keep track of the olde selection's column range for each group, as well
as extracting `find_next_columnar_selection_by_display_row` and
`find_next_columnar_selection_by_buffer_row` helper methods to clarify
the two navigation strategies.

Closes #46842

Release Notes:

- Fixed `editor::AddSelectionAbove` and `editor::AddSelectionBelow` when
skipping over soft-wrapped lines to preserve the original cursor column
when adding selections through lines shorter than the starting position