Autodetect sixel support with escape codes

[jedahan]

Instead of maintaining a list of TERM, something like

read -s -t 1 -d "c" -p $'\e[c' >&2
if [[ $REPLY =~ ";4;" ]]; then
  RenderTarget::Sixel
fi

from https://github.com/hackerb9/lsix/commit/5bddfe0c59f30f2d042f33a517a45317a45233e4