Replace collapsible list JavaScript with details/summary

[KateMorley]

This project incorporates my collapsible list JavaScript, which I created over a decade ago, before the native details/summary elements existed. You may want to consider switching to the native elements, for which I have a tutorial including suggested styling: https://iamkate.com/code/tree-views/

[svenevs]

Hey @KateRoseMorley! This is AWESOME, I very much like this, I know so little web stuff -- the post is really accessible 🙃

I'm sorry for the delay, I got an initial prototype, but not functional. I'm looking forward to this change, I think it will make exhale much more compatible with many more html themes. Thanks so much ❤️