Suggest installer optionally set ALTERNATE_EDITOR

[Joel Reicher]

Since the recommended way (in the W32 FAQ) to get file associations working is 
to set ALTERNATE_EDITOR (amongst other things) I think it would add value for 
the installer to offer this as an optional step.

In my opinion a nice default value is

<emacs root>\emacs-28.1\bin\runemacs.exe --no-splash

as I believe most users would expect the splash screen to be suppressed when 
launching emacs via a file association, but may still want the splash screen 
when launching emacs directly.

It also seems to me quite benign to set an environment variable that probably 
isn't used by anything else.

An additional option to add

(server-start)

to an init file (creating if necessary) would also be nice, but perhaps a 
little less benign.

Thoughts?

Thanks and regards,

       - Joel