pass fromEl and toEl to hook beforeUpdate

[SteffenDE]

Relates to #3516.
Relates to #3615.

Allowing beforeUpdate to cancel an update by returning false is not implemented, as this would require more complex internal changes.

[probably-not]

@SteffenDE what's the status of this PR? I have a package that implements FLIP animations in a hook (LiveFLIP), and having the fromEl and toEl in the beforeUpdate hook callback would be a huge benefit to avoid jittering issues.

[SteffenDE]

@probably-not I wanted to get some feedback from @chrismccord first.. Apart from that I'd need to add some tests. I did not forget about this, so the goal is to release it in LiveView 1.1 if Chris is happy with it.

You can try this change with

{:phoenix_live_view, github: "phoenixframework/phoenix_live_view", branch: "sd-beforeUpdate-assets", override: true}

and let me know if it works for you :)

[probably-not]

Thanks @SteffenDE! I'll try it out from the branch and see how things are working 🙏