Implement more YEARLY RRULE stanza options

[s0600204]

The changes encapsulated within this PR add support for the BYYEARDAY, BYWEEKNO and BYSETPOS recurrence rule stanzas - and expands on the existing BYDAY stanza support - when any of these are used with the YEARLY frequency.

Also included are relevant tests.

After this PR, only BYMONTHDAY[^1] and the time-based stanzas (BYHOUR, BYMINUTE, BYSECOND) are not supported under YEARLY.

Refs. #11

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[^1] - A future PR for this is in the works, but touches more than just YEARLY.

[u01jmg3]

Thanks for this addition and I appreciate the rundown on how to interpret the spec.

Happy for you to be named on the list of contributors if you want to be added :)

[u01jmg3]

@s0600204: can you comment on support for BYWEEKDAY?

[s0600204]

Happy for you to be named on the list of contributors if you want to be added :)

Sure.

Can you comment on support for BYWEEKDAY?

BYWEEKDAY isn't part of the RFC5545 spec, it appears to be an alias (of sorts) for BYDAY that jakubroztocil's rrule.js (and the python-dateutil python module it's based on) uses.

The usage example in the README for rrule.js shows a byweekday property being passed to the RRule constructor, and when the .toString() method is called on the created object, it has been translated to a BYDAY. Similarly, it appears that a BYDAY within the RRULE of an event is made available on the byweekday property when parsed by either of those parsers.