[Suggestion] Underline styles and colors

[rysson]

What about underline styled (dotted, curly etc) and its colors?
I know it's an extension but supported by popular emulators, Kitty for example.
Underline colors are implemented in Kitty, VTE, mintty, and iTerm2.

I could prepare same pull request if you are interested.
Some preview.

BTW. What reason is using RgbFg/RgbBg and EightbitFg/EightbitBg instead of just Rgb and Eightbit?
They are keys in the *Register but never together.