Add QuoteReply

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

+package net.mamoe.mirai.message.data
+
+/**
+ * 消息源, 用于被引用. 它将由协议模块实现为 `MessageSourceImpl`
+ */
+interface MessageSource : Message {
+    companion object : Message.Key<MessageSource>
+}
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mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/data/QuoteReply.kt

+package net.mamoe.mirai.message.data
+
+
+/**
+ * 群内的引用回复. 它将由协议模块实现为 `QuoteReplyImpl`
+ */
+interface QuoteReply : Message {
+    val source: MessageSource
+
+    companion object Key : Message.Key<QuoteReply>
+}
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