message: add String for SourceType

message/source.go

@@ -10,6 +10,21 @@ const (
 	SourceGuildDirect
 )
 
+func (t SourceType) String() string {
+	switch t {
+	case SourcePrivate:
+		return "私聊"
+	case SourceGroup:
+		return "群聊"
+	case SourceGuildChannel:
+		return "频道"
+	case SourceGuildDirect:
+		return "频道私聊"
+	default:
+		return "unknown"
+	}
+}
+
 // Source 消息来源
 type Source struct {
 	SourceType  SourceType