fix!: only handle promise input when opted in

[harlan-zw]

🔗 Linked issue

#368

❓ 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)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Promise support was initially added to Unhead to support lazy resolved file paths using Vite imports.

useServerHead({
  link: [
    {
      // promises supported
      href: import('~/assets/MyFont.css?url'),
      rel: 'stylesheet',
      type: 'text/css'
    }
  ]
})

While this feature did somewhat work it only did for the above example and was not used as far as I'm aware.

Promises add a lot of overhead in terms of performance optimization and extra weight to handle. To simplify the on-going maintenance of the package we drop support for them.

⚠️ Breaking Change

If you have promises as input they will no longer be resolved, either await the promise before passing it along or register the optional promises plugin.

Option 1: Await Promise

useHead({
  link: [
    {
-     href: import('~/assets/MyFont.css?url'),
+     href: await import('~/assets/MyFont.css?url'),
      rel: 'stylesheet',
      type: 'text/css'
    }
  ]
})

Option 2: Promise Plugin

import { PromisePlugin } from 'unhead/optionalPlugins'

const unhead = createHead({
  plugins: [PromisePlugin]
})