Support `RRULE` with frequency set to `YEARLY`

[maciektr]

Currently, while parsing recurrence rule :invalid_frequency is returned for any rule with frequency YEARLY, although it is supported by the standard.

cocktail/lib/cocktail/parser/i_calendar.ex

Lines 209 to 216 in 2b12aa0

	@spec parse_frequency(String.t()) :: {:ok, Cocktail.frequency()} | {:error, :invalid_frequency}
	defp parse_frequency("MONTHLY"), do: {:ok, :monthly}
	defp parse_frequency("WEEKLY"), do: {:ok, :weekly}
	defp parse_frequency("DAILY"), do: {:ok, :daily}
	defp parse_frequency("HOURLY"), do: {:ok, :hourly}
	defp parse_frequency("MINUTELY"), do: {:ok, :minutely}
	defp parse_frequency("SECONDLY"), do: {:ok, :secondly}
	defp parse_frequency(_), do: {:error, :invalid_frequency}

https://datatracker.ietf.org/doc/html/rfc5545#section-3.3.10

       freq        = "SECONDLY" / "MINUTELY" / "HOURLY" / "DAILY"
                   / "WEEKLY" / "MONTHLY" / "YEARLY"

[vanvoljg]

@maciektr That's a good point! There are a places for improvement in our logic, and we would welcome a PR which adds this functionality, provided it's well-tested and doesn't break any existing usage (this package is used in multiple production apps).

I should have some time to work on updating this soon--but probably not this month.