Clear and concise description of the problem

i would like to embed code from files into fenced blocks:

for this i need some async action:
markdown-it/markdown-it#256 (comment)

i need to call my transform function on the parsed tokens array -
and then call the render function.

so would be super nice to be able to just insert async transformative calls between the two tasks..
something like:

const tokens = md.parse(your_source_md, env);
// run async function with only side-effects on tokens
await callAsyncFN(tokens, idx, options, env, sefl);
return md.renderer.render(tokens, md.options, env);

as fare as i can tell this would also mean an addition to the markdown-it-async wrapper.
(as there is only the full render call available at this..)

Suggested solution

add option similar to this
types.ts

// ...
/**
   * Custom tranformations apply before and after the markdown transformation
   */
  transforms?: {
    before?: (code: string, id: string) => string | Promise<string>
    renderingAfterParsingBeforeRender?: (tokens: array, idx: number, options: object, env: object, self:MarkdownIt) => None  | Promise<none>
    after?: (code: string, id: string) => string | Promise<string>
    /**
     * Return extra code to be injected into the `<script>` tag
     */
    extraScripts?: (frontmatter: Record<string, any>, id: string) => string[] | Promise<string[]>
  }
// ...

Alternative

No response

Additional context

i will try and create some sort of minimal example in the next days

for a complex test-setup you can have a look at
https://github.com/s-light/quasar-unplugin-vue-markdown-tests
https://codesandbox.io/p/github/s-light/quasar-unplugin-vue-markdown-tests/

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