Component: Scrollspy

**Original title:** Highlight TOC title for current heading in viewport

**Is your feature request related to a problem? Please describe.**
It would be nice if the table-of-contents component could highlight the title of the current heading in viewport.

**Describe the solution you'd like**
As suggested by @cferdinandi a separated component `scrollspy` could be implemented. The component could interact with the TOC maybe through events (?).

**Describe alternatives you've considered**
Time ago I developed a [component](https://git.andreafazzi.eu/andrea/tableofcontents) like this but I think we can produce a more robust alternative.

Start by locating the existing table-of-contents component and reviewing the linked reference implementation. Clarify how a separate scrollspy component should interact with the TOC, including whether events are needed. Done means the title for the heading currently in the viewport is highlighted.

Written by the indexing model from the issue text.