implement NACK range parsing in ACK frames

ackhandler/nack_range.go

@@ -0,0 +1,9 @@
+package ackhandler
+
+import "github.com/lucas-clemente/quic-go/protocol"
+
+// NackRange is a NACK range
+type NackRange struct {
+	FirstPacketNumber protocol.PacketNumber
+	Length            uint8
+}