[MDX] Switch from Shiki Twoslash -> Astro Markdown highlighter

[bholmesdev]

Bu-bu-but why? I like Twoslash!

We do too! Still, we've noticed a fair share of MDX + Shiki-twoslash issues being reported due to inconsistencies with Markdown. Namely:

• 3 support threads wondering about the injected code language at the top of each code block
• 1 issue on passing through Shiki Twoslash config via markdown.shikiConfig (which could involve breaking changes to resolve) Validation is preventing me from using shikiConfig.themes #4245
• Inconsistencies in CSS class targeting, where MDX files generate class="shiki" while Markdown generates class="astro-code"

Due to this, we think bundling Twoslash by default has more cons than pros. We also don't feel we can migrate Markdown to Twoslash now that we've reached 1.0 stability.

Can I still use Twoslash?

Absolutely! If you like Shiki-twoslash, you can flip off our built-in highlighter and apply their remark plugin like so:

import shikiTwoslash from 'remark-shiki-twoslash';

export default {
  markdown: {
	syntaxHighlight: false,
        remarkPlugins: [shikiTwoslash], // use twoslash in .md
  },
  integrations: [mdx({
	remarkPlugins: [shikiTwoslash],  // use twoslash in .mdx
  })],

Changes

• Replace Twoslash with Astro's existing Shiki parser
• Always apply the rehypeRaw plugin. This makes custom syntax highlighters easier to apply, and is consistent with our Markdown defaults.

Testing

• Update Shiki tests to correct class naming
• Add Shiki-Twoslash test

Docs

Add "custom highlighter" section to README

[changeset-bot]

🦋 Changeset detected

Latest commit: 67669ec

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
@astrojs/mdx	Minor
@e2e/preact-component	Patch
@e2e/react-component	Patch
@e2e/solid-component	Patch
@e2e/svelte-component	Patch

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[hippotastic]

Had a test-related nit, but other than that, LGTM.