i3: Add bindings to switch forward & backward

After unplugging an external monitor I sometimes still have old
workspaces that I can't access through their index.

.config/i3/config

@@ -108,6 +108,10 @@ bindsym $mod+0 exec ~/.config/i3/multi-monitor-workspaces.sh -s 10
 workspace_auto_back_and_forth yes
 bindsym $mod+Tab workspace back_and_forth
 
+# switch workspaces forward and backward
+bindsym $mod+Next workspace next
+bindsym $mod+Prior workspace prev
+
 # Switch visible workspaces (e.g. multi monitor setup)
 bindsym $mod+Shift+Tab exec i3-msg workspace "$( \
 	i3-msg -t get_workspaces | \