Allow specifying additional environment variables

[samvv]

I see that start-cosmic contains these environment variables:

export XDG_CURRENT_DESKTOP="${XDG_CURRENT_DESKTOP:=COSMIC}"
export XDG_SESSION_TYPE="${XDG_SESSION_TYPE:=wayland}"
export _JAVA_AWT_WM_NONREPARENTING=1
export GDK_BACKEND=wayland,x11
export MOZ_ENABLE_WAYLAND=1
export QT_QPA_PLATFORM="wayland;xcb"
export QT_AUTO_SCREEN_SCALE_FACTOR=1
export QT_ENABLE_HIGHDPI_SCALING=1
export DCONF_PROFILE=/usr/share/dconf/profile/cosmic

I am missing a TERM variable, as described in my other issue in cosmic-greeter. Currently it is set to linux at login, which I am told is really bad, and among other things breaks alternate screen mode in GNOME Console.

I propose adding a separate file where user-defined environment variables can be added and which are sourced into the start-cosmic if the file exists. This is different from adding them to ~/.profile, because xterm-256color should only be used in an environment where this is supported (like COSMIC).

At the very least, this problem appears on a fairly vanilla install of Arch Linux.