Update syntax-highlighting.server.mdx

[godmar]

add a note about the necessity of ESM for rehype.

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[ChristianMurphy]

This generally applies to all remark and rehype plugins. (Since unifiedjs/unified#121)
Not just rehype-highlight, it may make sense to add the note to https://mdxjs.com/docs/getting-started/#nextjs

[godmar]

This is the first time I've used the "edit" on github feature. I thought it would be a low overhead way to propose a change - turns out though that github just forked your repo and added a branch in my account! Anyway, I added another commit to my proposal.

[wooorm]

I don’t think this is the place to explain how to use ESM for MDX with each tool, or that all plugins are ESM. It would become a giant section, which is very similar to the existing Getting started or Extending MDX articles.

How about instead using ESM for the example in Getting started: https://mdxjs.com/docs/getting-started/#nextjs

[godmar]

Then please do it yourself.

I had an afterthought on this: the next.js starter, as of today, doesn't create a next.config.mjs file. So for people using the next.js starter, all the documentation for the rehypePlugins etc. is out of date from the get-go.

[wooorm]

mdx-js/.github#29