Project: color
Task (challenging): - add element `.color.hsl-wheel` to `.root`
- draw a hsl color wheel (class `wheel`) canvas in the `.hsl-wheel`
- canvas has the same width and height, color wheel fill the entire canvas
- lightness is 50
- red is at the leftmost point of the color wheel, with red, green and blue arranged clockwise
- saturated red is the left point of the canvas, unsaturated red is the center point of the canvas