User implemented mapping methods

If Mapperly cannot generate a mapping, one can implement it manually simply by providing a method body in the mapper declaration:

[Mapper]
public partial class CarMapper
{
    public partial CarDto CarToCarDto(Car car);

    private int TimeSpanToHours(TimeSpan t) => t.Hours;
}

Whenever Mapperly needs a mapping from TimeSpan to int inside the CarMapper implementation, it will use the provided implementation.