Add phx-custom-events to allow custom events to be handled by LiveView

[superchris]

This PR adds support for a phx-custom-events attribute which will cause a private hook to be added to listen for Custom Events.

The hook will use the value for this attribute which should contain either a single or comma-delimited list of custom event names, which will be pushed to LV. The detail property of the custom event, merged with the element dataset, will be used as the payload of the event sent to LV.

Alternatively, if the phx-custom-events attribute is present to trigger this hook, the hook will look for any attributes which begin with phx-custom-event- and allow the user to rename the event e. g. phx-custom-event-foo="bar"

This code originated in the phoenix-custom-event-hook project and has been used on several projects.

[SteffenDE]

Just as a heads up: please don’t include the built assets (in priv) in your PR. I’ll have a closer look in the next days :)

[SteffenDE]

I very much appreciate the inclusion of an e2e test! I briefly skimmed over the code and it looks quite reasonable. That being said, I still don't really like the use of a magic separator for specifying multiple events (see also my initial comment here https://elixirforum.com/t/support-handling-customevents-in-liveview/64896/6). I don't have a clearly better solution though.

@chrismccord had some thoughts on this as well iirc; one important thing was the handling on non-serializable data in the event detail.