Closes #13905

This PR fixes columnar selection to originate from mouse position
instead of current cursor position. Now columnar selection behaves as
same as Sublime Text.

1. Columnar selection from click-and-drag on text (New):


https://github.com/user-attachments/assets/f2e721f4-109f-4d81-a25b-8534065bfb37

2. Columnar selection from click-and-drag on empty space (New): 


https://github.com/user-attachments/assets/c2bb02e9-c006-4193-8d76-097233a47a3c

3. Multi cursors at end of line when no interecting text found (New): 


https://github.com/user-attachments/assets/e47d5ab3-0b5f-4e55-81b3-dfe450f149b5

4. Converting normal selection to columnar selection (Existing):


https://github.com/user-attachments/assets/e5715679-ebae-4f5a-ad17-d29864e14e1e


Release Notes:

- Fixed the issue where the columnar selection (`opt+shift`) incorrectly
used the cursor to start the selection instead of the mouse position.