support for understyles, for example curly

[adamyg]

Terminfo:
Smulx=\E[4::%p1%dm, aka Cs='\e[4:3m", Ce='\e[4:0m",
and style+color, Setulc=\E[58\:\:2\:\:%p1%{65536}%/%d\:\:%p1%{256}%/%{255}%&%d\:\:%p1%{255}%&%d%;m
Conpty support, detection problem: microsoft/terminal#7434

Examples:

\e[4:3mtest\e[0m
\e[4:3;58:2::000:255:000mtest\e[0m

Additional features, SGR 58 and 59 sequences specify the color of the underline. That is, 58;5;idx for an entry of the 256-color palette, or 58;2;r;g;b for direct RGB.

[adamyg]

build-28