Create mitt like event hub for event_hub_factory

What does this MR do?

This is part of the effort to decouple ourselves from the Vue instance as an event bus. This functionality will not be available in Vue 3. Unfortunately simply importing something like mitt will not work because there are some API quirks we are still dependent on like multiple event args.

Okay Paul, so what's the plan?

• This MR, which "forks mitt" so that we gain the benefit of not creating a new Vue instance for every event hub.
• #223863 (closed) Add a eslint rule to disallow calling $emit with multiple args
• &3334 (closed) Clean up our existing code
• #223864 (closed) Then we can probably adopt something like mitt