vim: change cursor shape when only one cursor

In vim mode, change the cursor shape depending on the mode. This is only
applicable if `enable_terminal_cursor` is set to true and there is only
one cursor in the editor.

src/tui/mode/input/vim/visual.zig

@@ -37,6 +37,7 @@ pub fn create(a: Allocator) !tui.Mode {
         .description = "vim",
         .line_numbers = if (tui.current().config.vim_visual_gutter_line_numbers_relative) .relative else .absolute,
         .keybind_hints = &hints,
+        .cursor_shape = .underline,
     };
 }