Allow Link fields to be restricted to only internal or external URLs

Description of the need

If you want to add a Link field to a content type for storing URLs to external websites (e.g. like on the BackdropCMS.org Showcase nodes), users can still enter URLs for internal pages. Additionally, there's an autocomplete that displays a list of matching pages/content on your site (you may not want users seeing this list of content in this situation).

Proposed solution

It'd be nice if there was an option for Link fields to restrict them to either external or internal URLs (or both, as is the situation currently). Then you wouldn't have the autocomplete showing when you only wanted users entering external URLs.

Alternatives that have been considered

Drupal has the Advanced Link contrib module that achieves this, however it also does a bunch of other things too.

Additional information

Here's a screencast showing the issue on Backdrop's Showcase node form:

Draft of feature description for Press Release (1 paragraph at most)

Backdrop now optionally allows Link fields to be restricted to just external or internal URLs.