Create an ExtensionSet to handle logic around multiple extensions

[mmastrac]

There is a fair amount of logic in JsRuntime that deals with collating the data from multiple Extensions. We should move all of this logic to an ExtensionSet that can be initialized from a slice of extensions.

In addition, the ExtensionSet should be extendable to add more extensions to it at a later time. This will allow us to build a base ExtensionSet for the snapshot, then extend it when loading further extensions (for example, for the worker and CLI).

Finally, investigate if we can or should persist the ExtensionSet as part of the snapshot for the purpose of verifying that the correct extensions were passed in as part of snapshot init, or potentially to avoid having to re-create the set of Extensions for the base snapshot.