Support highlighting codespan using highlight option + or support async function in Renderer

[aminya]

Describe the feature

1. Currently, marked only allows specifying a highlight function for code blocks and not inline code (codespan).

2. Using the custom Renderer is not possible because (new Renderer()).codespan does not accept async functions. Thus, there is no way to use an async highlighter for highlighting codespan unless an exoteric solution like deasync is used.

Why is this feature necessary?
To highlight inline code.

Describe alternatives you've considered
Using a Pandoc renderer. However, lang can be provided externally through other parts of the code, and so this is independent of supporting Pandoc syntax for giving the language through the markdown syntax.

[UziTech]

duplicate of #458 PRs are welcome 😁

[aminya]

Well, this had two sections. The first one can be simpler I think since the highlight function already exists for code.

[UziTech]

The issue with highlighting codespans is that you can't tell marked what language it is because there can't be an infostring like a code block.

You could always do the highlighting after running the markdown through marked.

[aminya]

I can by using a lambda. For example, here I don't care about "lang" at all. I just supply my own "scopeName" (language)

https://github.com/atom-ide-community/atom-ide-markdown-service/blob/d02dce38781de4d1a71a0e3c84c0c146aff5f49c/src/renderer.ts#L72

[UziTech]

You could use a walkTokens function:

const walkTokens = (token) => {
  if (token.type === 'codespan') {
    token.text = highlight(token.text);
  }
};

marked.use({ walkTokens });

For an async walk tokens function you will have to await all of the promises.

const tokens = marked.lexer(md, opts);
const promises = [];
marked.walkTokens(tokens, (token) => {
  if (token.type === 'codespan') {
    promises.push(asyncFunction(token));
  }
});
await Promise.all(promises);
const html = marked.parser(tokens, opts);