www/zine-ssg: Add port

Zine is a static site generator similar in concept to www/gohugo.

Zine turns your content into HTML, styles it using your layouts, and finally
copies the result into an output directory that you can then publish on static
hosting services like GitHub Pages.

Zine has a companion port, devel/superhtml which acts as an LSP for the markdown
content and HTML layouts.

Differential Revision: https://reviews.freebsd.org/D55877