Curly and colored underlines

This is originally a feature of Kitty, now also adopted by VTE (GNOME Terminal and friends). Technically two separate features, but they mostly make sense together, e.g. for spell checking.

Apparently vim and neovim have already / are about to support these, see e.g. vim undercurl, vim color, neovim.

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The new SGR 4:3 (\e[4:3m) attribute, strictly with a colon as separator, was introduced to start a curly underline.

In the mean time, 4:0, 4:1 and 4:2 were also added as aliases for the standard 24 (turn off all kinds of underlining), 4 (single underline) and 21 (double underline), respectively.

At some point in the future, probably 4:4 and 4:5 could also stand for dotted and dashed underlines in some order (these are the five types of underlining supported by HTML/CSS).

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The new SGR 58 and 59 sequences specify the color of the underline, following the pattern of 38 and 39. That is, 58;5;idx or 58:5:idx for an entry of the 256-color palette, or 58;2;r;g;b or 58:2:... (wait, see #6377 (closed)) for direct RGB. There's no shortcut notation for the first 16 entries (corresponding to SGR 30-37 and 90-97), use the 256-color mode with indices of 0-15 instead.

59 reverts to the default, that is, the underline's color auto-following the text color.

In case you're short of bits, I believe it's okay to drop some precision, e.g. store only 4 bits per color channel. We were also considering this in the VTE bug.