Make `kvk` extension private

[rakuyoMo]

I think KVKCalendar is mainly a calendar framework, so is it necessary to set the system-type extension tools listed below to public? I think users are likely to have their own calendar tools.

For example, the KVKDequeueProxyProtocol protocol itself has exceeded the scope of the calendar. Is it necessary to set the protocol and the corresponding extension method to public?

Here are the results after I did some simple searching: