Format highlighting as DocLayout Docs using ANSI styles

[silby]

As observed in, jgm/pandoc#9565 (comment), skylighting-format-ansi as-is produces somewhat bad results when rendering with a background color different from the terminal default, which is quite visible in the context of the work-in-progress Pandoc ANSI writer. We could control the output a bit better in that context by adding a formatter that returns a doclayout Doc. The goal would be a Doc that when rendered with doclayout's renderANSI would have the style's background color limited to a rectangle bounding the code block.

High fidelity to highlight styles will require support in doclayout for indexed and 24 bit colors, jgm/doclayout#32