Selection in emacs mode includes one additional character

[Jiogo18]

Platform Windows 10, Arch Linux
Terminal software Windows Terminal, Konsole, Alacritty

With Emacs edit mode, the first shift+left/right select 2 characters.
Git bisect found #867, specifically editor.rs.
It seems Vi mode and Emacs mode should behave differently here.

Steps to reproduce

1. cargo run --example basic or nushell with $env.config.edit_mode = "emacs"
2. Write text 0123456789
3. Select text with shift+left or shift+right
4. An additional character to the right is selected

Screenshots/Screencaptures

https://github.com/user-attachments/assets/b44f8646-9c08-4a70-94da-afe55a390adc
Keys:

• shift+right, right
• delete
• undo
• shift+left, left
• delete
• undo

[collinmurch]

I implemented & tested #927, which fixes this described issue.