feat(Currency.js): get default currency

This commit introduces several changes in bin/Currency.js to make the code more readable and consistent:

1. Improved function declaration style: Rather than using function(arg1, arg2) { .. }, we now use function (arg1, arg2) { .. } for better style consistency.
2. Cleaned up unnecessary whitespace: Removed redundant lines to make the code cleaner and more readable.
3. Added a new function: Introduced a new function getDefaultCurrency() for fetching the default system currency. This asynchronous function returns a Promise that resolves to the code of the default currency.