fix: extra char when vim option 'selection' = exclusive

[adoyle-h]

Description

When :set selection=exclusive, the returned value from get_visual() will contain an extra char.

Use vim.fn.getregion which follows vim option 'selection' to fix this issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Configuration:

• Neovim version (nvim --version): v0.10.1
• Operating system and version: Darwin Kernel Version 23.6.0

Checklist:

• [x] My code follows the style guidelines of this project (stylua)
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation (lua annotations)