`MailboxData.searchSort` should be rolled into `.search`

[danieleggert]

This is https://www.rfc-editor.org/rfc/rfc4551#section-3.5 which allows for a server response such as

SEARCH 1 2 3 (MODSEQ 4)

Instead of its own case, this should probably just be part of

public enum MailboxData: Hashable {
    …
    case search([UnknownMessageIdentifier], ModificationSequenceValue? = nil)
    …
}

and
MailboxData.SearchSort should be removed.