more CSS Color syntax variants

[Crissov]

• percentage values for RGB
• percentage for alpha, <alpha-value> = <number> | <percentage>
• explicit angle units for hue, <hue> = <number> | <angle>
• RGB and HSL notation with alpha value but without a in function name
• RGB and HSL notation without commas (and with slash preceding optional alpha value), CSS Color level 4
• 4-digit and 8-digit hexadecimal RGB codes
• transparent is a transparent black
• some case insensitivity tests

[louwers]

@kkaefer I know this is an old repo, but you merge this one? 😇