Support AtAll in message.first()

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/data/AtAll.kt

@@ -14,7 +14,7 @@ package net.mamoe.mirai.message.data
  *
  * @see At at 单个群成员
  */
-object AtAll : Message {
+object AtAll : Message, Message.Key<AtAll> {
     override fun toString(): String = "@全体成员"
 
     // 自动为消息补充 " "

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/data/MessageChain.kt

@@ -155,6 +155,7 @@ inline fun <reified M : Message> MessageChain.any(): Boolean = this.any { it is
 @Suppress("UNCHECKED_CAST")
 fun <M : Message> MessageChain.firstOrNull(key: Message.Key<M>): M? = when (key) {
     At -> first<At>()
+    AtAll -> first<AtAll>()
     PlainText -> first<PlainText>()
     Image -> first<Image>()
     Face -> first<Face>()