phoneValidator function
Implementation
String? phoneValidator(String? pn,{String invalid="invalid number!",bool cn=false,bool us=true}){
if(pn==null)return invalid;
final ppn=unformatPhone(pn,trim1: false);
if(ppn.startsWith("1")){
if((cn||us)&&ppn.length==11)return null;}
else if(us&&ppn.length==10) {return null;}
return invalid;
}