add hsl() to types/color (#16322)

types/color/color-tests.ts

@@ -59,3 +59,4 @@ var chain: Color.Color = color
     .opaquer(0)
     .rotate(0)
     .mix(colorOther, 0)
+    .hsl();

types/color/index.d.ts

@@ -85,6 +85,7 @@ declare namespace Color {
         opaquer(value: number): Color;
         rotate(value: number): Color;
         mix(color: Color, value?: number): Color;
+        hsl(): Color;
     }
 }