Support for ColorScheme

Mentioned way back in https://github.com/cli/go-gh/issues/5#issuecomment-1110563550, it would be great if `ColorScheme` could be migrated from `cli/cli`'s `IOStreams`. I was going to start a fork since I find myself repeating much of this often, but realize with `x/color` you might have something in mind. If there's a design in place, I'd love to take that into account and make sure it's threaded through all the places here currently like `tableprinter` and, at least to some degree, `x/markdown` — though, seems that will probably need more extensible support or at least a more general RGB color value, which is hard to make theme-aware generically.

Start by reviewing the ColorScheme implementation in cli/cli's IOStreams and the existing x/color direction, then inspect tableprinter and x/markdown. Done means the design is settled and ColorScheme support is threaded through those areas, including extensible or RGB-aware needs identified for markdown.

Written by the indexing model from the issue text.