isvalidmod10number function: this objective of this function is to take a 16 digit credit card number as a string parameter and create an integer array of each digit of extracted from the credit card number. then, starting from the last digit of the number array, multiply alternate number by 2 based on a check bit. if the multiplied number is greater than 9, subtract 9 from it and re-set the digit. if the multiplied number is less than or equal to 9, new digit will be the multiplied value. finally, add the digit to a final sum. return value of the function is the totalsum mod 10.