Add support for new .NET 6 types

[govert]

From this discussion: https://groups.google.com/g/exceldna/c/JtWnyEyJeM0

• Support for ValueTask<T> return on ExcelFunction. (Need to convert ValueTask to Task using the "AsTask()" method) • Include DateOnly (native struct on .net 6.0) in the marshalling. (There would be no need to convert a DateOnly to other types)