New port: editors/xi-term

Formely known as xi-tui, xi-term is a terminal frontend for the xi editor.

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.
• 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/xi-frontend/xi-term

PR: 237533
Submitted by: Nuno Teixeira <ed.arrakis@gmail.com>
Reviewed by: tobik
Differential Revision: https://reviews.freebsd.org/D20159