Add Sixel Support

[ShaunLawrie]

Is your feature request related to a problem? Please describe.
No problem.

Describe the solution you'd like
I'd like to be able to render sixel images with Spectre.Console e.g.

Describe alternatives you've considered
It's like my third sixel rendering approach, the first few were too slow and more complex. The docs for sixel are not great so there isn't a correct way to approach a lot of this.

Additional context

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