feat(webpack, vite): default to .js extension for client

[danielroe]

🔗 Linked issue

#6232 (comment)
https://github.com/nuxt/framework/discussions/6168

❓ 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

Now that we have updated vue-bundle-renderer, we can output.js files on client-side, for better support with CDN/web servers.

This PR also allows users to configure their output filenames on vite. (Previously it always overrode them.)

import { defineNuxtConfig } from 'nuxt'

export default defineNuxtConfig({
  vite: {
    build: {
      rollupOptions: {
        output: {
          chunkFileNames: '_nuxt/[hash].mjs'
        }
      }
    }
  }
})

📝 Checklist

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

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