editors/xi-core: Update to latest snapshot

• Submitter adopts the port
• Add DOCS option to provide some supplementary info
• The project also moved to a new repository thus update GH_ACCOUNT and the WWW field

Changelog:

https://github.com/xi-editor/xi-editor/compare/3164cb6...c16973f

PR: 237532
Submitted by: Nuno Teixeira <ed.arrakis@gmail.com>
Reviewed by: tobik
Differential Revision: https://reviews.freebsd.org/D20158