feat: add slider widget

[rtgiskard]

For me, this makes it possible to control backlight with ddcutil:

"slider#backlight": {
	"label": "󰃟",
	"max_limit": 86,
	"min_limit": 1,
	"min": 0,
	"max": 100,
	"cmd_setter": "~/.config/hypr/script/utils.sh swaync_backlight set $value",
	"cmd_getter": "~/.config/hypr/script/utils.sh swaync_backlight get"
},

swaync_backlight() { # args: set|get [value]
	case "$1" in
	set) ddcutil setvcp 0x10 $2 ;;
	get) ddcutil getvcp 0x10 | grep -oP "current value =\s+\K[0-9]+" ;;
	esac
}