Add postal code pattern for Monaco

https://en.wikipedia.org/wiki/Postal_codes_in_France#Monaco

Author: moogblob

src/lib/isPostalCode.js

@@ -50,6 +50,7 @@ const patterns = {
   LU: fourDigit,
   LV: /^LV\-\d{4}$/,
   LK: fiveDigit,
+  MC: /^980\d{2}$/,
   MG: threeDigit,
   MX: fiveDigit,
   MT: /^[A-Za-z]{3}\s{0,1}\d{4}$/,