custom cursor shape

I want to use `beam` shape in prompt. It automatically works if I run lazygit inside kitty (maybe also need to set default cursor shape in kitty config). But not in neovim's embeded terminal. This is a quick patch for me.

```diff
diff --git a/pkg/gui/gui.go b/pkg/gui/gui.go
index 358c284f5..1e70c3a05 100644
--- a/pkg/gui/gui.go
+++ b/pkg/gui/gui.go
@@ -766,6 +766,7 @@ func (gui *Gui) initGocui(headless bool, test integrationTypes.IntegrationTest)
Width: width,
Height: height,
})
+ gocui.Screen.SetCursorStyle(6)
if err != nil {
return nil, err
}
```