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:

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

### Code

```js
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
}
}
```