Add currency number list function

[chris-jamieson]

In some situations, for instance when providing a list of ISO currency numbers for a validation schema (e.g. via Joi), it is really useful to simply be able to fetch a list of the currency numbers.

This PR adds a similar function to the existing codes() and countries() functions, called numbers(), which returns an array of the ISO numbers as strings.

[freeall]

This looks good! One tiny thing, I'm ok with the tabs into spaces, but can you remove the ' -> " change? Then I'll merge it.

[chris-jamieson]

@freeall thanks for taking a look and sorry for the slow response. I'm embarrassed to say I didn't even realise my editor's prettier plugin had made those changes - doh! My apologies.

Have reverted the double-quotes to single-quotes now. Thank you.

[freeall]

@jamiesoncj published as 1.2.0. Thanks for this!