textproc/cast2gif: Add port 0.1.0 pre-release

Tool to render Asciinema cast files to GIFs without
using Electron or a web browser

The port contains the pre-release [1] plus two commits,
added since then.

[1] https://github.com/katharostech/cast2gif/releases

PR: 251064
Submitted by: Nuno Teixeira <ed.arrakis@gmail.com> (maintainer)
Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D27435