Add IsCurrency to StringExtensions.

Continuing from [Currency validation support #4430](https://github.com/CommunityToolkit/WindowsCommunityToolkit/pull/4430#issuecomment-997855061).

We'd like to add an `IsCurrency` extension method to [StringExtensions](https://github.com/CommunityToolkit/dotnet/blob/main/CommunityToolkit.Common/Extensions/StringExtensions.cs) that bundles an easy and obvious check for if a string is a valid currency, similar to [IsDecimal](https://github.com/CommunityToolkit/dotnet/blob/main/CommunityToolkit.Common/Extensions/StringExtensions.cs#L66).

This would be implemented like so
```cs

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