Add MessageSource.quote

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

@@ -14,6 +14,8 @@ package net.mamoe.mirai.message.data
  * 消息源只用于 [QuoteReply]
  *
  * `mirai-core-qqandroid`: `net.mamoe.mirai.qqandroid.message.MessageSourceFromMsg`
+ *
+ * @see MessageSource.quote 引用这条消息, 创建 [MessageChain]
  */
 interface MessageSource : Message {
     companion object : Message.Key<MessageSource>

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

@@ -33,4 +33,13 @@ fun MessageChain.quote(sender: Member): MessageChain {
         return QuoteReply(it) + sender.at() + " " // required
     }
     error("cannot find MessageSource")
+}
+
+/**
+ * 引用这条消息.
+ * 返回 `[QuoteReply] + [At] + [PlainText]`(必要的结构)
+ */
+fun MessageSource.quote(sender: Member): MessageChain {
+    @UseExperimental(MiraiInternalAPI::class)
+    return QuoteReply(this) + sender.at() + " " // required
 }