change highlight color from an RGB value to `Base.error_color()`

[KristofferC]

RGB colors have very spotty support in terminals so probably better to use something more well-supported. Base.error_color() is by default :light_red but can also be customized with an env var.

Ref https://github.com/JuliaLang/JuliaSyntax.jl/issues/61

Edit: Okay I see there is this whole machinery for using RGBs so I guess this was a conscious choice. Waiting with this until there is a decision on what to do.

[c42f]

Yeah _printstyled() is my own machinery because Base isn't very flexible, so the base color codes won't work here.

RGB colors have very spotty support in terminals so probably better to use something more well-supported

I used RGB to satisfy my own desire for things to be pretty and to have sane mappings to color codes. But for general use we'll need something widely supported :+1:

Unfortunately Base.printstyled can't do background colors I think? And it can't do reverse until Julia 1.7. So we probably still need a little of our own machinery here.

So we "just" need to figure out what's generally supported and do that. Can we use 255 colors for example, or do we need to limit ourselves to 16? Is reverse color better supported than background colors?