Create a function that takes a string t and a number n and returns the repeated string n number of times.
Return "Not a string!" if given argument t is not a string,
repeatString("Tesha", 2) ➞ "TeshaTesha"
repeatString("Deep", 3) ➞ "DeepDeepDeep"
repeatString("Matt", 4) ➞ "MattMattMattMatt"
repeatString(2000, 7) ➞ "Not a string!"