New port: editors/xi-core

The xi editor project is an attempt to build a high quality text
editor, using modern software engineering techniques.

Goals include:

• Incredibly high performance. All editing operations should commit and paint in under 16ms. The editor should never make you wait for anything.
• Beauty. The editor should fit well on a modern desktop, and not look like a throwback from the '80s or '90s. Text drawing should be done with the best technology available (Core Text on Mac, DirectWrite on Windows, etc.), and support Unicode fully.
• Reliability. Crashing, hanging, or losing work should never happen.
• Developer friendliness. It should be easy to customize xi editor, whether by adding plug-ins or hacking on the core.

WWW: https://github.com/google/xi-editor

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D11162