Add ability to clone HBS instance

This would allow us to register partials and helpers on a global instance and clone it to modify the environment for another use case.

Start by reading the existing HBS instance and environment APIs, then inspect how partials and helpers are registered and how related behavior is tested. Define what cloning must preserve and how the clone can be modified independently; done means a global instance can be cloned for another use case without changing the original.

Written by the indexing model from the issue text.