Format highlighting as DocLayout Docs using ANSI styles

As observed in, https://github.com/jgm/pandoc/pull/9565#issuecomment-2016610872, 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, https://github.com/jgm/doclayout/issues/32

Start by reviewing skylighting-format-ansi and the Pandoc ANSI writer context described in the issue. Check doclayout's renderANSI and doclayout issue #32 for the indexed and 24-bit color limitations. Done means a formatter returns a doclayout Doc whose rendered ANSI background is bounded to the code block while preserving supported highlight styles.

Written by the indexing model from the issue text.