TOC Tree Component, TocItems array on publication state

[bluefirepatrick]

This draft PR adds a modal dialog for the TOC using the beta react-aria-components Tree control. It also creates a TocItem array -a hierarchical plain objects array with unique ids for each toc item -and stores this array on the publication state.

This book has a multi-level TOC that is useful for testing:
https://github.com/readium/go-toolkit/blob/develop/test/feedbooks_book_6816.epub

Any changes/comments are welcomed.

[JayPanoz]

As a quick heads up, branch action-refinements will be merged into develop with most actions disabled as they’re not currently functional so that the main deployment page can benefit from other improvements + new showcase publications. And start work on theming from develop.

I tested this branch and checked the conflicts to make sure they are pretty trivial to resolve. I don’t forecast dramatic changes that will impact this draft PR so we should be able to change the target from action-refinements to develop w/o creating an issue further down the line.

I’m not forgetting I owe you an in-depth review, @bluefirepatrick 🙏