fix(vite): external modules under/esm/ pattern

[antfu]

Co-authored-by: Ben bencodezen@users.noreply.github.com

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Found during pairing with @bencodezen 🙌

To reproduce the bug (Vite):

<script setup lang="ts">
import format from 'date-fns/format'

const date = format(new Date(), 'yyyy/MM/dd')
</script>

<template>
  <div>
    {{ date }}
  </div>
</template>

which will cause the error of export is not a valid syntax. Searching down, the root cause is that the internal module of date-fns been externalized but in a .js format of ESM in server.mjs

This PR adds the conversion to externalize modules under /esm/ which I believe is a common pattern (date-fns is using is at least).

https://unpkg.com/browse/date-fns@2.25.0/esm/_lib/protectedTokens/index.js

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[pi0]

Also made unjs/mlly#23 to resolve for mlly/externality