HexValidation - Allow 4-digit hex values

Similar to #214, 4- and 8-digit hex codes are a W3C standard (meaning `#RGBA` and `#RRGGBBAA`, respectively). The linter should allow these, perhaps with an optional toggle flag.

Further reading: [W3C](https://drafts.csswg.org/css-color/#hex-notation) | [MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/color_value#RGB_colors)