Refactor MDX postprocess plugin

[bluwy]

Changes

Refactor the @astrojs/mdx-postprocess Vite plugin to its own file and separate its inner content to functions. This makes the code a bit more readable, and prepares for some upcoming changes I'd like to add to this plugin.

I had a few iterations of this refactor locally. Initially using MagicString so that we can prepend strings with sourcemap intact, however I noticed it has a significant perf impact, so I switched back to plain strings. Here's the Rollup build time of the Astro docs.

• Main: 113s
• MagicString w/ sourcemap: 132s
• MagicString w/o sourcemap: 111s
• Plain strings (this PR): 110s

Testing

Existing tests should pass.

Docs

Internal refactor. No changeset.

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Latest commit: cbe9122

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[ematipico]

I don't understand. Does this PR move code, or does it change too? If it does change the code, could you please tell us where the changes are? Since you moved the file, it's very difficult to view the change in logic

[bluwy]

It only moves the code. There shouldn't be any logic change, other than how it updates the code string in the transform hook.

[ematipico]

Thank you, I was confused by the perf comparison

[bluwy]

Ah yeah it was to show there's no perf regressions 😅