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.