Add IsCurrency to StringExtensions.

[Arlodotexe]

Continuing from Currency validation support #4430.

We'd like to add an IsCurrency extension method to StringExtensions that bundles an easy and obvious check for if a string is a valid currency, similar to IsDecimal.

This would be implemented like so

public static bool IsCurrency([NotNullWhen(true)] this string? str)
{
    return decimal.TryParse(str, NumberStyles.Currency, out _);
}