onHsl method
Creates chalk with background color defined from HSL (Hue, Saturation and Lightness) color space parameters. https://en.wikipedia.org/wiki/HSL_and_HSV
Implementation
Chalk onHsl(num hue, num saturation, num lightness) {
var rgb = ColorUtils.hslTorgb(hue, saturation, lightness);
return makeRGBChalk(rgb[0], rgb[1], rgb[2], bg: true);
}