Bring back silky-smooth nav animations

As of this commit

- https://github.com/diffplug/gitfromscratch/commit/f6539968ef687ac3af20019786c76bc983e21a3e
- https://7b215f1c.gitfromscratch.pages.dev/branches/sticky-notes-and-paintbrushes

The nav did a cool animation to subitems on a page.

It has problems because the items can now be multi-line, the arithmetic for where to move the background highlight is tricky.

So we removed it in

- https://github.com/diffplug/gitfromscratch/pull/17
- https://e5d5819c.gitfromscratch.pages.dev/

It would be cool to add it back someday, but it's not high-priority.

Start by comparing the navigation behavior in commit f6539968ef687ac3af20019786c76bc983e21a3e and its preview with the state after pull request #17. Trace the existing navigation implementation and account for multi-line subitems; done means the subitem background highlight animates smoothly without incorrect positioning.

索引モデルが issue の本文から書いたものです。