Represents any of the possible Message objects.
t() :: LineBot.Message.Text.t() | LineBot.Message.Sticker.t() | LineBot.Message.Image.t() | LineBot.Message.Video.t() | LineBot.Message.Audio.t() | LineBot.Message.Location.t() | LineBot.Message.Flex.t() | LineBot.Message.Imagemap.t() | LineBot.Message.Template.t()