Selection in VI visual mode need to include the last character (+1 grapheme to the right)

[blindFS]

Platform macOS Terminal software wezterm

https://github.com/user-attachments/assets/57347516-eeed-441e-b1b8-02cb1f1111de

This is not what a traditional vim behavior where current character "a" should be included in both directions. Basically, the end offset returned by get_selection should be increased to the next grapheme.

Steps to reproduce

1. set editor mode to vi
2. press v to visual selection

Seems that crate::core_editor::Editor should be aware of current edit mode in order to fix this issue.