add support for Sixel

[IanButterworth]

I think this is ok as a direct dep?
Also, needs testing.

If REFERENCETESTS_SIXEL=1

[oxinabox]

Sixel actually adds a 50 direct dependencies, including ImageCore, which we currently use a hack to avoid depending on directly.
With that said maybe we should just depend on it (and ImageCore and XImageInTerm) directly,
making maintenance and use easier.

If we are depending on it directly, i think we don't need to use a enviroment variable to decide if to use it or not, and can just use Sixel.is_sixel_supported() and if so always use it. Its always better, no?

[IanButterworth]

Unfortunately that returns false inside the Pkg.test worker in a local terminal where it otherwise returns true.

[oxinabox]

Oh thats annoying we should open an issue on Sixel.jl about that