Colored and styled underlines

|kitty| supports colored and styled (wavy) underlines. This is of particular use in terminal based text editors such as :program:vim and :program:emacs to display red, wavy underlines under mis-spelled words and/or syntax errors. This is done by re-purposing some SGR escape codes that are not used in modern terminals (CSI codes <https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_(Control_Sequence_Introducer)_sequences>__)

To set the underline style::

<ESC>[4:0m  # no underline
<ESC>[4:1m  # straight underline
<ESC>[4:2m  # double underline
<ESC>[4:3m  # curly underline
<ESC>[4:4m  # dotted underline
<ESC>[4:5m  # dashed underline
<ESC>[4m    # straight underline (for backwards compat)
<ESC>[24m   # no underline (for backwards compat)

To set the underline color (this is reserved and as far as I can tell not actually used for anything)::

<ESC>[58...m

This works exactly like the codes 38, 48 that are used to set foreground and background color respectively.

To reset the underline color (also previously reserved and unused)::

<ESC>[59m

The underline color must remain the same under reverse video, if it has a color, if not, it should follow the foreground color.

To detect support for this feature in a terminal emulator, query the terminfo database for the Su boolean capability.