Support Prism.js grammar parsing but use the Highlight.js HTML/theme pipeline

Related #2212.

I'm not sure this belongs in core yet, and I don't necessarily want a hard or soft Prism dependency. I suppose perhaps you could also pass Prism into the function itself though so it's late binding?

Caveats

• Using custom parsers is still private API since the Emitter API is public yet. Hence the __emitTokens name of the property.
• See #3620

What needs to be done

• Figure out where this goes, how to package it, etc.
• Write a comprehensive prismTypeToScope function
  • Ref: https://github.com/highlightjs/highlight.js/issues/2212#issuecomment-1243663341
  • Ref: https://prismjs.com/tokens.html

Example Usage

To replace our JavaScript support with Prism's:

import Prism from 'prismjs'
const prismJS = fromPrism(Prism.languages.javascript)
hljs.registerLanguage("javascript", prismJS)

Code

function prismTypeToScope(name) {
  // very poor results, must be improved
  return name;
}

function prismTokensToEmitter(tokens, emitter) {
  tokens.forEach(token => {
    if (typeof(token) === "string") {
      emitter.addText(token)
    } else if (token.type) {
      let scope = prismTypeToScope(token.type)
      if (typeof(token.content) === "string") {
        emitter.addKeyword(token.content, scope)
      } else { // array of tokens
        emitter.openNode(scope)
        prismTokensToEmitter(token.content, emitter)
        emitter.closeNode()
      }
    }
  })
}

function prismParserWrapper(code, emitter) {
  const tokens = Prism.tokenize(code, this._prism)
  prismTokensToEmitter(tokens, emitter)
}

function fromPrism(prismGrammar) {
  return function(hljs) {
    let self = {}

    Object.assign(self, {
      _prism: prismGrammar,
      __emitTokens: prismParserWrapper.bind(self)
    })
    return self
  }
}