codecs update

codec/packet/login/server/packet.go

@@ -0,0 +1,19 @@
+package server
+
+import (
+	"github.com/Tnze/go-mc/data/packetid"
+	pk "github.com/Tnze/go-mc/net/packet"
+)
+
+type ServerboundPacket interface {
+	pk.Field
+	PacketID() packetid.ServerboundPacketID
+}
+
+type serverPacketCreator func() ServerboundPacket
+
+var packetRegistry = make(map[packetid.ServerboundPacketID]serverPacketCreator)
+
+func registerPacket(id packetid.ServerboundPacketID, creator serverPacketCreator) {
+	packetRegistry[id] = creator
+}