Port BYSETPOS with YEARLY from libical

[ray007]

Testing across platforms, one of our test events looks like this:

BEGIN:VEVENT
UID:[email protected]
DTSTAMP:20221115T100439Z
DTSTART:20221226T110000
DTEND:20221226T130000
SUMMARY:YearTest
PRIORITY:9
RRULE:FREQ=YEARLY;BYDAY=MO;BYSETPOS=1,-1
END:VEVENT

which should give me an event every first and last monday every year.
But gives me an event every monday.

[kewisch]

It appears BYSETPOS with YEARLY is not currently implemented. It does seem to be implemented in a stock libical, so it might be worth checking if the code can be ported.