feat(session): import XDG_SESSION_DESKTOP and DESKTOP_SESSION

The variables are useful for running terminal emulator servers as a
systemd units (`foot --server`). Shell profile scripts may depend on
the variables normally present in the desktop session.

Fixes #16

src/session.sh

@@ -38,8 +38,10 @@
 #  - https://systemd.io/DESKTOP_ENVIRONMENTS/
 #
 export XDG_CURRENT_DESKTOP=sway
+export XDG_SESSION_DESKTOP=${XDG_SESSION_DESKTOP:-sway}
 export XDG_SESSION_TYPE=wayland
-VARIABLES="DISPLAY I3SOCK SWAYSOCK WAYLAND_DISPLAY XDG_CURRENT_DESKTOP XDG_SESSION_TYPE"
+VARIABLES="DESKTOP_SESSION XDG_CURRENT_DESKTOP XDG_SESSION_DESKTOP XDG_SESSION_TYPE"
+VARIABLES="${VARIABLES} DISPLAY I3SOCK SWAYSOCK WAYLAND_DISPLAY"
 SESSION_TARGET="sway-session.target"
 WITH_CLEANUP=""