requireLanguage deprecation

https://github.com/highlightjs/highlight.js/pull/2844#pullrequestreview-529833532

Something you should look out for in the future... may want to switch back to getLanguage or write your own if you need the specific "blow up" error behavior. We're recommending build-time dependencies now rather than runtime... IE you'd use a ES6 module module to just import the dependency. See `javascript.js` and `ecmascript.js`, for examples.

As long as you're using the 3rd party build process you should be able to do this also.