feat: add IsTextMention to MessageEntity

[niuhuan]

This works when we tag a member without a username, It is different from a mention.

for _, entity := range message.Entities {
    if entity.IsTextMention() {
        println(entity.User.ID)
    }
    if entity.IsMention() {
        println(message.Text[entity.Offset:entity.Offset+entity.Length])
    }
}