Migrate data to "Link" field

LinkIt has been a great plugin over the years, but with Craft 5's improved Link field we don't have a strong reason to use it in most instances anymore.

However, the plugin isn't configured to migrate its content back to the standard Link field, but it would be excellent if there was a CLI like `craft linkit/convert-to-link` (and vice-versa, for folks upgrading to LinkIt).