home: wm: i3: launch tmux when starting terminal

Similarly to the `zsh` snippet, launch tmux unless I explicitly do not
want it.

The reason I also add it in `i3` is to make the launch happen sooner,
and to avoid doing double work on some shell startup shenanigans if I
can.

home/wm/i3/default.nix

@@ -157,7 +157,9 @@ in
         keybindings = lib.my.recursiveMerge [
           {
             # The basics
-            "${modifier}+Return" = "exec ${terminal}";
+            "${modifier}+Return" = "exec ${terminal} ${
+              lib.optionalString config.my.home.tmux.enable "-e tmux new-session"
+            }";
             "${modifier}+Shift+Return" = "exec env TMUX=nil ${terminal}";
             "${modifier}+Shift+q" = "kill";
             "${modifier}+f" = "fullscreen toggle";