added sixel encoding

[BaerLKR]

This is in regards to this #38 issue, as suggested i used icy_sixel

It encodes to sixel if it finds sixel in the TERM environment variable. That sadly isn't always the case but I couldn't find another way to detect if a terminal supports sixel. And I wanted to prevent false-positive sixel output because if you don't know what it is this string of random chars is rather confusing. So there will be quite a few false-negatives but there is still the half block fallback from viuer

I hope this is what y'all had in mind and I am looking forward to your criticism

[kaikalii]

I think you need to use option_env!

[BaerLKR]

yeah i just noticed too
will fix in just a second

[BaerLKR]

I think you need to use option_env!

I think I need to use std::env::var because it is a runtime check