highlightjs getter

[yhatt]

Expose highlight.js instance through get highlightjs() getter to make languages easy extensible.

// engine.js for Marp CLI
import highlightjsHLSL from 'highlightjs-hlsl'

export default ({ marp }) => {
  marp.highlightjs.registerLanguage('hlsl', highlightjsHLSL)
  return marp
})

And now Marp Core generates an independent highlight.js instances per Marp Core instances. It's important for the app using Marp Core, to avoid share the instance between highlight.js for Marp and the highlight.js for other purpose.