Support sixel protocol

[wookayin]

Add a Sixel backend for imgcat. This is a widely adopted protocol for displaying graphics. One may use libsixel (and its python binding) or direct implementation (akin to lsix) if we value portability more.

Terminal requirements and compatibility list:

• Sixel compatible terminals: XTerm, MLTerm, MinTTY/cygwin, st, iTerm2 (3.0+)
• Non-compatible: libvte (terminator, gnome-terminal)

[manueldeprada]

+1 for this. VScode is sixel compatible, and a lot of us use a fork of Alacritty that implements sixel support.