COLORTERM=lilyterm is not detected by programs, we should set to “truecolor”

[Profpatsch]

LilyTerm/src/notebook.c

Line 167 in faf1254

g_string_append_printf (environ_str, "\tCOLORTERM=lilyterm");

COLORTERM is a semi-standardized variable to signal the program whether the terminal emulator has true color support.

See alacritty/alacritty#1526
and the linked writeup https://gist.github.com/XVilka/8346728.

@mueslix researched a lot into these things, and he said: “>90% set the variables to "truecolor" (konsole, gnome-terminal, iterm, kitty, alacritty, .....)”

cc @morhetz who added this in 8e53a99

[Profpatsch]

Hm, setting it to truecolor makes the colors appear very weird in fish, so maybe lilyterm does not come with truecolor support.

I set it to true instead, and now it works for applications that expect it (they can set it to 256 colors).

Update: ah, lilyterm seems to use an old vte version, pre-truecolor. I still think we should set it to true.