Support conversion to/from DateOnly and TimeOnly

.NET 6 Preview 4 added DateOnly and TimeOnly structs. I would like to propose that Noda Time should support two-way conversion between DateOnly and LocalDate, and between TimeOnly and LocalTime.

public struct LocalDate
{
    public static LocalDate FromDateOnly(DateOnly date);
    public static LocalDate FromDateOnly(DateOnly date, CalendarSystem calendar);
    public DateOnly ToDateOnly();
}

public struct LocalTime
{
    public static LocalTime FromTimeOnly(TimeOnly time);
    public TimeOnly ToTimeOnly();
}

[mattjohnsonpint]

I can probably live with the multi-targeting if we have to…

I think that’s the right move.

[0xced]

I already have started experimenting with adding a net6.0 target, you can have a look at my net6.0-DateOnly-TimeOnly branch.

Currently, I have added the .NET 6.0 target framework and fixed all the nullability warnings that ensued. And in my last commit, I just implemented public static LocalDate FromDateOnly(DateOnly date) and an extension method on DateOnly: public static LocalDate ToLocalDate(this DateOnly date) => LocalDate.FromDateOnly(date);

Now, many tests (1691) are failing on .NET 6.0 (same root cause related to building the time zone map) but that should probably be tracked in another issue since it’s completely unrelated to the new DateOnly and TimeOnly types.