Make classes sealed: `MessageEvent`, `UserMessageEvent`, `GroupAwareMessageEvent`

mirai-core-api/src/commonMain/kotlin/event/events/MessageEvent.kt

@@ -30,7 +30,7 @@ import net.mamoe.mirai.utils.MiraiInternalApi
  *
  * @see isContextIdenticalWith 判断语境相同
  */
-public interface MessageEvent : Event, Packet, BotPassiveEvent { // TODO: 2021/1/11 Make sealed interface in Kotlin 1.5
+public sealed interface MessageEvent : Event, Packet, BotPassiveEvent {
     /**
      * 与这个消息事件相关的 [Bot]
      */
@@ -85,7 +85,7 @@ public interface MessageEvent : Event, Packet, BotPassiveEvent { // TODO: 2021/1
  * @see FriendMessageEvent
  * @see GroupTempMessageEvent
  */
-public interface UserMessageEvent : MessageEvent {
+public sealed interface UserMessageEvent : MessageEvent {
     public override val subject: User
 }
 
@@ -149,7 +149,7 @@ public class OtherClientMessageEvent constructor(
  * @see FriendMessageEvent
  * @see GroupTempMessageEvent
  */
-public interface GroupAwareMessageEvent : MessageEvent {
+public sealed interface GroupAwareMessageEvent : MessageEvent {
     public val group: Group
 }