Make Transit an optional dependency

[Deraen]

Transit format is now included in the default options and required on the core ns. AOT compiling Muuntaja core will also compile the Transit library.

User could exclude the dependency and add a local namespace to override muuntaja.format.transit with no-op format declarations. Even this is somewhat inconvenient as those formats need to return Encoder/Decoder instances for the default instance.

Transit isn't a big dependency, but in some environments, like Azure Function/AWS Lambda, everything extra on the classpath makes the startup slower.

We could separate the core and some easy ns which includes options/instance with Transit and others.