Patch ColorTag

[kechuan]

filter unformat HEXColor to prevent FormatException crash.

my Example:

BBCodeText(
          data: 
          "[color=#ffffff]test[/color] \n" 
          "[color=ffffff]test[/color] \n" 
          "[color=33ffffff]test[/color]" //typo
          "[color=RED]test[/color]", //otherReason
          stylesheet: BBStylesheet(
            tags: [PatchColorTag()],
          )
        )
        
 ...
//colorTag
 if(hexColorRegExp.hasMatch(hexColor)){
    debugPrint("match it: $hexColor");
    return oldStyle.copyWith(color: HexColor.fromHex(hexColor));
  }
        

output:
flutter: match it: #ffffff
flutter: match it: ffffff