Use StyledStrings

[sethaxen]

StyledStrings is in Julia v1.11 part of the standard library, but it can be installed separately for older Julia versions. It would allows us to define named "faces" (styles) for the different k-hat categories, which dependents could then use. I think in principle this would allow our colors to appear in HTML and Markdown representations, though currently not many packages have implemented support for styled strings.

Unless we can figure out a way to make this support an extension, on v1.10 and older, having StyledStrings as a direct dependency will increase package load times by ~1s.

[sethaxen]

The issues with load time are known, and we should hold off depending on StyledStrings at least until there's a solution to this: JuliaLang/julia#57998