misc/emacs-libvterm: New port for Emacs libvterm integration

Emacs-libvterm (vterm) is fully-fledged terminal emulator inside GNU
Emacs based on libvterm, a C library. As a result of using compiled
code (instead of elisp), emacs-libvterm is fully capable, fast, and it
can seamlessly handle large outputs.

Reviewed by: jrm
Pull Request: https://github.com/freebsd/freebsd-ports/pull/480