Add warning if you use styling in settings sheet

[yanokwa]

Collect and Enketo do not support styling in form_title, form_id, version, and instance_name. It's not an unreasonable thing to want, but if it's easy to detect, we should warn (or error?) that styling is not yet supported in those places.

https://forum.getodk.org/t/adding-linebreaks-and-bold-text-in-instance-name-in-form-settings/39559