Make Honeybadger.setNotifier an abstract method

[subzero10]

With the recent introduction of notifier per package, we want to take advantage of Typescript and make sure we set a new notifier identifier for every new js client integration.
This can be done by modifying the setNotifier method to be abstract.
With this approach, we should:

• Have a class extending the base class for each client integration (i.e. HonebyadgerReactClient, HoneybadgerVueClient, etc). The extending classes could extend of the HoneybadgerJsClient (not the core client). These classes will implement the setNotifier() method.
• Challenge: The @honeybadger-io/js package should not export a singleton, but rather the class itself so it can be extended and instantiated by the target package (vue, react, etc.)