Implement `x-cms-preview`, adding support for preview `strings`.

[julrich]

Those can be interpreted differently by different CMS implementations. In Storyblok it needs to support preview templates and preview fields.

[julrich]

Some work for Storyblok was already done here:

• 610de66
• 4a12f96

[julrich]

Netlify Create support for such a feature is currently unclear. Seems to always take either a) a static string (which doesn't help) or b) automatically uses the first string typed field in the controls of the previewed component.