[FR] Rework floating TOC feature to remove tocify.js dependency

[cderv]

From https://github.com/rstudio/rmarkdown/issues/2405 and related to it.

I think the much worse thing is that tocify.js is no longer maintained, and we are still using a version from 7 years ago...

Maybe we should move from tocify.js and recreate using Lua filter + HTML / JS / CSS. Not trivial work but could be worth doing since unmaintained anyway

If we manage to offer similar feature using another library or custom JS / HTML / CSS, that would be helpful for the future.