Don't consider hex colour values of length 4 or 8 as invalid

Hex colours can be 3, 4, 6 or 8 characters in length which is already handled in Color.FromHex but ColorToken.IsValid was only considering lengths 3 and 6 as valid and so these hex colours would get rejected

Author: lee-m

src/ExCSS/Tokens/ColorToken.cs

@@ -7,7 +7,7 @@ public ColorToken(string data, TextPosition position)
         {
         }
 
-        public bool IsValid => Data.Length != 3 && Data.Length != 6;
+        public bool IsValid => Data.Length != 3 && Data.Length != 4 && Data.Length != 6 && Data.Length != 8;
 
         public override string ToValue()
         {