☂️ Refactor `JObject` hierarchy

[HosseinYousefi]

In preparation for dart-lang/sdk#55062, we need to change the structure of JObject to be composed of a JReference instead of inheriting one. This makes it more natural, where a JObject is a JReference and a JType and the JReference itself is responsible for its life cycle and not the JObject (See #970 (comment)).

This potentially enables us to have multiple kinds of JReferences like JGlobalReference and JWeakGlobalReference or a global reference that is not managed by a native finalizer.

For version 0.8.0, we can land these changes all at once:

• Change the hierarchy of JObject
• Wrap JMethodIDPtr and JFieldIDPtr into high level classes #985
• Enable sharing of JObjects across isolates #1054
• Remove JObject generic methods like callMethodByName #549