Detect truecolor support in kmscon

[rdvdev2]

A patched version of kmscon for true color compatibility reports true color support by setting COLORTERM to kmscon. This PR allows helix to recognize it.

[the-mikedavis]

I think this should be changed in kmscon or in your /etc/environment instead of in Helix.

COLORTERM is not formally defined anywhere as far as I can tell but the usual scheme is to test for truecolor. Some tools also check 24bit:

• https://github.com/termstandard/colors/tree/2179a60be353e8739dc8cd0afa7f153ebd7e5714#checking-for-colorterm
• Neovim: https://neovim.io/doc/user/term.html#tui-colors
• Emacs: https://github.com/emacs-mirror/emacs/blob/ad5a67996ddf23df904c09165475759e2e0a68b1/doc/misc/efaq.texi#L969
• st: https://www.mail-archive.com/hackers@suckless.org/msg11307.html
• Alacritty: Proposal: Set COLORTERM variable to advertise 24-bit support alacritty/alacritty#1527
• S-Lang library (used by Midnight Commander): http://lists.jedsoft.org/lists/slang-users/2016/0000014.html
• Nim language runtime library: https://github.com/nim-lang/Nim/blob/82d80e6e2cb09652e15d8ec81621bba734872aaa/lib/pure/terminal.nim#L937-L938
• https://jdebp.uk/Softwares/nosh/guide/TerminalCapabilities.html

Gnome terminal is a notable exception: it removed the COLORTERM var and it was gnome-terminal beforehand. Also see https://bugs.launchpad.net/ubuntu/+source/gnome-terminal/+bug/1429584 - maybe there's a better way to do the detection with terminfo? Last I saw on that was that the terminfo maintainer was not interested in detecting truecolor support via terminfo but that source is a little old at this point. This gnome-terminal bug filing has some interesting discussion pointing out that it could be possible to use xtermcontrol or checking the VTE_VERSION (which we already do for extended underline detection).

The issue for adding this in kmscon is here: dvdhrm/kmscon#60. I'm not sure why they chose the kmscon value. Using COLORTERM like TERM_PROGRAM is not ideal as any client program (like Helix) needs to add every compatible terminal to its detection list.

[rdvdev2]

I will then open a PR in kmscon for this. Thank you!