bug#48439: 28.0.50; Support for the Alacritty terminal emulator

[Eli Zaretskii <eliz@gnu.org>]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: tsdh@gnu.org,  48439@debbugs.gnu.org
> Date: Sat, 15 May 2021 15:43:57 +0200
> 
> On Mai 15 2021, Eli Zaretskii wrote:
> 
> > Looks quite extensive to me.  Of particular importance are: the number
> > of colors (not 256 but only 8 in alacritty), and different commands to
> > set fore- and back-ground colors.
> 
> Both are overridden in the alacritty description.

OK, thanks.  Then I guess Alacritty is indeed close enough to
xterm-256color.  The only question I have is: does Alacritty offer any
capabilities that are not in xterm-256color?  Because if it does, then
adding it to the list of aliases will prevent users from taking
advantage of those additional capabilities, because AFAIU we use the
alias without checking first whether the original terminal name has a
library.  So if Alacritty has important additional features, it might
be better to prepare a separate alacritty.el file, using xterm.el as a
starting point (or even 'require'ing it).