Factory to build encode and decode functions to share encoders and decoders #56

I'm thinking about a factory function that creates { encode, decode } functions, just like as https://github.com/mcollina/msgpack5

The interface is something like this:

import { factory } from "@msgpack/msgpack";
const { encode, decode } = factory({
  extensionCodec, // common options
  encodeOptions, // encoder options
  decodeOptions, // decoder options
});

The factory pattern is also adopted by msgpack-ruby, which is designed by the primary author of MessagePack.