Create a function that takes a string $txt and a number $n and returns the repeated string $n number of times.
If given argument $txt is not a string, return Not A String !!
repeatString("Mubashir", 2) ➞ "MubashirMubashir"
repeatString("Matt", 3) ➞ "MattMattMatt"
repeatString(1990, 7) ➞ "Not A String !!"