[11.x] feat: introduce option to change default Number currency

[sts-ryan-holton]

Hi 👋,

This PR introduces the functionality to globally change the default currency of the Number helper, much like the locale. Right now the default is passed aa a function argument, however, it can be useful to change the global currency.

The use case here is that the default currency is still USD for backwards compatibility with applications, and the $in variable second arg is still retained.

I've introduced a $currency variable, defaulted to US.

[kapersoft]

I would handle it the same as the $locale-parameter:

    public static function currency(int|float $number, ?string $in = null, ?string $locale = null)
    {
        static::ensureIntlExtensionIsInstalled();

        $formatter = new NumberFormatter($locale ?? static::$locale, NumberFormatter::CURRENCY);

        return $formatter->formatCurrency($number, $in ?? static::$currency);

[sts-ryan-holton]

I went with ! empty() in case $in was passed as empty :)