~/.config/sway: Add keybindings for controlling brightness

.config/sway/config

    bindsym XF86AudioMute        exec pactl set-sink-mute   @DEFAULT_SINK@ toggle
    bindsym XF86AudioPlay        exec playerctl play-pause
    bindsym XF86MonBrightnessDown exec brightnessctl set 5%-
    bindsym XF86MonBrightnessUp   exec brightnessctl set 5%+
    bindsym $mod+Shift+f exec ~/.bin/toggle-fan
    bindsym $mod+Shift+s exec grimshot save area