Add common Flag.Keyword instances

[danieleggert]

public enum Keyword: Equatable {
    /// `$Forwarded`
    case forwarded
    /// `$Junk`
    case junk
    /// `$NotJunk`
    case notJunk
    /// `Redirected`
    case unregistered_redirected
    /// `Forwarded`
    case unregistered_forwarded
    /// `Junk`
    case unregistered_junk
    /// `NotJunk`
    case unregistered_notJunk
    /// `$MailFlagBit0`
    case colorBit0
    /// `$MailFlagBit1`
    case colorBit1
    /// `$MailFlagBit2`
    case colorBit2
    /// `$MDNSent`
    case mdnSent
    case other(String)
}