add undercurl as a style

[dankamongmen]

At least kitty and wezterm, and probably others, support undercurl. Add NCSTYLE_UNDERCURL and the necessary code to support it, where available.

[dankamongmen]

it looks like both wezterm and kitty are using Smulx=\E[4\:%p1%dm, and alacritty also has this defined, though not in the terminfo database entry i have for it, odd.

kovidgoyal/kitty#3725
wez/wezterm#415

[dankamongmen]

So it looks like underline and undercurl (and double underline) are mutually exclusive.

That's actually good, because we want to be able to color the underline etc., which there's no way to do through the set_styles() interface. BUT we can add a function to configure the current underline state...except where would we then put this in rendered mode? it makes sense as a per-nccell thing...but we only have 4 bits free in the nccell.

hrmmmm. hrmmmmmmmmmmm.

for direct mode, it's trivial, since we just need a single current state, which we just pass through.

[dankamongmen]

[WSLUser]

Not relevant at the moment but for awareness Windows Terminal supports underline and double underline. There's also a request to get all the Kitty extended underlines supported including undercurl: microsoft/terminal#7228

[dankamongmen]

sweet! i hope to be compiling on Windows very soon.

does microsoft terminal run on anything but windows? if so, i can add support for it now.

[WSLUser]

No, most of the VT support comes from the open console (aka conhost). Most of the code should be portable to Windows Terminal though. Also you'd need to use a different UI framework first or use something like Uno (I would be interested in seeing this particular Terminal become x-plat but MS can't support doing that, especially when there's still gaps in VT to fill still). Don't want to contaminate this issue further with anything unrelated.