Event listeners for plugin lifecycle methods

### Feature Proposal

Event listeners for plugin lifecycle methods. Rather than needing to create a "standardized" plugin object which lives within the chart ecosystem, you could easily just attach some listeners to an instance of a plugin in certain situations, and remove them imperatively. This would make it much easier to have UI separate from the chart which reflects data of the chart (ex. a custom React legend, which shows all the datasets and allows toggling).

Today it seems like chart.platform.addEventListener supports things like clicks and mouse moves, but not plugin lifecycle methods like "afterUpdate"

### Possible Implementation

Simply exposing a function `chart.addEventListener('afterUpdate', fn)` allowing users to add listeners.

This could be hacked today by adding a custom plugin which attaches an "events" object to Chart instances, allowing callers to hook into any of the plugins methods.