term: add functions for detecting Sixel support + example

[larpon]

This adds 2 functions (and an example) to the term module that can query the terminal's graphics attributes.
One for detecting Sixel support and one for how many color registers the graphics device has in use. The later is useful if you later want to try to set a higher number of colors, depending on how many colors the Sixel image have.

Running the example in a Sixel compatible terminal should look something like this:

Konsole:

Xterm in vt340 mode:

Vanilla xterm needs this in ~/.Xresources:

xterm*decTerminalID: vt340
xterm*numColorRegisters: 256
xterm*sixelScrolling: 1
xterm*sixelScrollsRight: 1

To apply run: xrdb -merge ~/.Xresources

Terminal Sixel support is tracked here: https://www.arewesixelyet.com

ImageMagick can convert regular images to sixel format, if you want to play around:

convert image.png sixel:- > image.six # ImageMagick < v7
magick image.png sixel:- > image.six # ImageMagick >= v7

Bonus tip: it also works over ssh