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/tui.zig

@@ -717,6 +717,7 @@ pub const Mode = struct {
     description: []const u8,
     line_numbers: enum { absolute, relative } = .absolute,
     keybind_hints: ?*const KeybindHints = null,
+    cursor_shape: renderer.CursorShape = .block,
 
     fn deinit(self: *Mode) void {
         self.handler.deinit();