Feature Request: Color Conversion to 256, 16 color

[cyntheticfox]

So far, I really like Aura. I've been using it with Kitty for a few months, and would like to try to use it elsewhere, but in environments where I may be limited to only 256 colors a la xterm-256, or even only 16 colors. Some terminal applications after all are only written to use those.

Could we have one or more additional helper function(s) to approximate these? I think that would help in porting a lot.