Support Shiki's langAlias option in markdown configuration

[Sec-ant]

Body

Summary

Support Shiki's langAlias option in markdown configuration so we can register custom language aliases for the code blocks.

Background & Motivation

In our website we have some code blocks that are tagged with cjs, because they contain code that has cjs-specific syntax like "use strict", module.exports, .etc. However Shiki doesn't support syntax highlighting for cjs blocks out of box and will fallback to use txt:

• https://deploy-preview-610--biomejs.netlify.app/linter/rules/no-redundant-use-strict/
• https://app.netlify.com/sites/biomejs/deploys/666c5e50c6f7750008bb8102#L143-L156

Astro provides the langs option to register custom languages, but that also means we have to double load the js grammar for the cjs language like we did in biomejs/website#609 which is not very elegant.

On the other hand, there is a langAlias option in Shiki that allows registering aliases for a language, but it is not exposed from Astro.

Goals

Support langAlias option in markdown configuration.

Example

import { defineConfig } from 'astro/config';

export default defineConfig({
  markdown: {
    shikiConfig: {
      langAlias: { 
        cjs: "javascript",
      },
    },
  },
});

[bluwy]

Seems like an ok addition to me 👍

[ematipico]

This will be shipped in Astro v4.16