Closes #31648

This PR adds support for:
- Expanding multiple cursors above/below
- Expanding multiple selections above/below
- Adding new cursors/selections when expansion has already been done.
Existing expansions preserve their state and expand/shrink according to
the action, while new cursors/selections act like freshly created ones.

Tests for both cursor and selections:
- below/above cases
- undo/redo cases
- adding new cursors/selections with existing expansion

Before/After:


https://github.com/user-attachments/assets/d2fd556b-8972-4719-bd86-e633d42a1aa3


Release Notes:

- Improved `AddSelectionAbove` and `AddSelectionBelow` to extend
multiple cursors/selections.