Given a credit card number we can determine who the issuer/vendor is with a few basic knowns.
Complete the function `getIssuer()` that will use the values shown below to determine the card issuer for a given card number. If the number cannot be matched then the function should return the string `Unknown`.
| Card Type | Begins With | Number Length |
|------------|----------------------|---------------|
| AMEX | 34 or 37 | 15 |
| Discover | 6011 | 16 |
| Mastercard | 51, 52, 53, 54 or 55 | 16 |
| VISA | 4 | 13 or 16 |
getIssuer(4111111111111111) == "VISA"
getIssuer(4111111111111) == "VISA"
getIssuer(4012888888881881) == "VISA"
getIssuer(378282246310005) == "AMEX"
getIssuer(6011111111111117) == "Discover"
getIssuer(5105105105105100) == "Mastercard"
getIssuer(5105105105105106) == "Mastercard"
getIssuer(9111111111111111) == "Unknown"