Add support for new .NET 6 types

[govert]

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

• Support for ValueTask 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)

[Tommo56700]

TimeOnly would also be useful