Fix Compatibility with Immer Middleware

[joebobmiles]

See #53.

Immer invokes Object.freeze on non-primitive values such as arrays and objects. This is how it achieves immutability. This presents two problems for the Yjs middleware:

1. Client updates must obey Immer's rules for modifying objects and arrays (annoying but doable).
2. State updates triggered on the client by a peer need to either:
   a. Invoke Immer's produce function (unfavorable coupling), or
   b. Detect if the object to modify is frozen or otherwise inextensible and treat it as immutable.