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

[Profpatsch]

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https://github.com/Tetralet/LilyTerm/blob/faf1254f46049edfb1fd6e9191e78b1b23b9c51d/src/notebook.c#L167

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

See https://github.com/alacritty/alacritty/issues/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 https://github.com/Tetralet/LilyTerm/commit/8e53a9927450f041f524e0e86cfd05998edce2bc

[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.