Sixel performance improvements

[hpjansson]

@jjrv is working on an impressive sixel encoder that outperforms libsixel and may be outperforming ours. I'm pretty sure it already beats us on size, and it makes fewer passes, so it wouldn't surprise me if it's faster too.

Currently there's some discussion in #27 and a well-documented TypeScript implementation in https://gist.github.com/jjrv/69fbf5f1c8b49be070063bd2a1c1288f?ts=4 .

This issue is for discussion and improvements of sixel encoding approaches, both from Chafa's perspective and in general. Thanks @jjrv for bringing it up!

[jjrv]

The TypeScript package is now ready and published in a worthy location:

https://github.com/at-lib/sixel
https://www.npmjs.com/package/@lib/sixel