add highlight.js and tomorrow-night-eighties.css to comm-plat assets

This adds the `highlight.pack.js` lib (a special build for the languages we need -- https://highlightjs.org/download/) and the `tomorrow-night-eighties.css` syntax theme to the comm-plat assets to allow for Syntax Highlighting in the docs. 

This coincides with a PR to DuckDuckGo-Publisher (https://github.com/duckduckgo/duckduckgo-publisher/pull/127)

This seemed like the right place for the files to go, please let me know if these belong somewhere else.

//cc @sdougbrown @russellholt @jbarrett @jdorweiler