Allow naked anchor links

[admanami]

Please consider adding naked anchor links as an allowable url type.

e.g. #chapter-1

Currently these are rejected unless they begin with a fully qualified or relative URL. This creates a problem when previewing entries.

For use-cases where the URL is a link to a CTA on the same page, having a naked anchor link without needing to add a relative/full URL would make previewing/publishing easier, because the anchor link does not work in preview mode without the preview parameter and token, and these are unwanted in the URL once the entry is published.