Typing error in getSupportedCallingCodes returns an array of number, not an array of string

[jcchalte]

in the library Typescript index.ts file, the function ` getSupportedCallingCodes is declared as returning an array of string. But the underlying Google javascript function return an array of numbers, not an array of string, as declared here :

https://github.com/google/libphonenumber/blob/669751dd889a2daf135580caf5cb54f41fa0bc2d/javascript/i18n/phonenumbers/phonenumberutil.js#L1517

Should I create a simple PR with the getSupportedCallingCodes declaration corrected ?

Thank you for your awesome library, and have a great day !