feat: add server islands support for MDX

[apatel369]

Changes

reopening #12446 for main
Closes #12252

Server Islands are not rendering as islands in .mdx files because server islands metadata is not added for JSX components in MDX files.
Even after adding server islands metadata, the issue persists because transform function in vite-plugin-server-islands.ts file runs before server metadata is added in rehype.ts file.

I found that vite-plugin-server-islands.ts is added last in create-vite.ts and has enforce: 'post' but still transform function is running before the server meta data is added.

I am looking for feedback and guidance from Astro contributors on how to fix it.

Testing

TODO

Docs

Not needed

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Latest commit: 18449c8

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[codspeed-hq]

CodSpeed Performance Report

Merging #12574 will not alter performance

_{Comparing apatel369:arpan-#12252-mdx-islands (18449c8) with main (97f413f)}

Summary

✅ 6 untouched benchmarks