Add At.qq to provide a quick way to get the target

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/network/protocol/tim/packet/event/MessageEvent.kt

@@ -91,6 +91,8 @@ abstract class MessagePacketBase<TSubject : Contact> : EventPacket, BotEvent() {
     suspend inline fun Image.downloadAsByteArray(): ByteArray = getLink().downloadAsByteArray()
     suspend inline fun Image.download(): ByteReadPacket = getLink().download()
     // endregion
+
+    suspend inline fun At.qq(): QQ = bot.getQQ(this.target)
 }
 
 // region group message