[themes] Support for hue, saturation, lightness (HSL) color codes for theming

Support for hue, saturation, lightness (HSL) color codes for theme colors would allow for easier iteration in theme design by providing a more human readable format for color codes. I find it much easier to modify color values using HSL than it is with hex codes because the values being modified more closely model how humans think about color rather than how the computer renders color. Example: ```jsonc { "colors": { "editor.background": "hsla(301, 46%, 9%, 1)", // #220c21, } } ```