`createEncoder`, `createDecoder` and `DecodedMessage` are function/types
useful to the user so they should have easy access to it.
We still export `Decoder` and `Encoder` but in a path so it cam be
re-used by `@waku/message-encryption`.
Reasoning: by exposing the `Decoder` and `Encoder` classes to the user,
the user may care about them, try to use the method etc.
By "hiding" them away and providing `create*` help, the aim is for the
user to just call a function instead of instantiating a class.
Also, `V0` does not provide much information to the user so removing it.
