A11y `JS.push_focus` and `JS.pop_focus` do not work as expected with nested modals

[balexand]

I'm working on an app that has a modal rendered over another modal as well as a modal rendered over a side drawer.

Expected behavior

I would expect JS.push_focus and JS.pop_focus to act like push and pop operations in a stack in order to work correctly when opening one modal over another modal.

Actual behavior

The focusStack variable where the focused element is stored is not a stack but rather just a single element.

https://github.com/phoenixframework/phoenix_live_view/blob/37bcbc4515efbedd3e85b3c0fbd39c4f88cea9f7/assets/js/phoenix_live_view/js.js#L98-L107