Changes

SPIGOT-7804: Fix written book serialization

* Account for null/missing values when deserializing the 'resolved' and
  'generation' fields.
* Serialize the book pages as JSON strings.
* Avoid redundant conversion from strings to components to JSON and back to
  components during deserialization.
  Add CraftChatMessage.fromJSONOrString that accepts a maxLength argument
  and remove the no longer used fromJSONOrStringToJSON, fromStringToJSON,
  and fromJSONComponent helper methods.