feat: retry callback

[enkot]

🔗 Linked issue

#358

❓ Type of change

• 📖 Documentation (updates to the documentation, readme, or JSdoc annotations)
• 🐞 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)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds support for a callback for retry option, which is a function that takes a fetch context object and count of retries and returns a boolean (true if the request needs to be retried):

await $fetch('/cart', {
  retry: (ctx, count) => {
    return count <= 3 && ctx.error?.code === '007'
  }
})

It runs onError and after a response, so the developer can decide when the request needs to be retried.

📝 Checklist

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

[gustavopch]

My 2 cents: shouldRetry seems to better describe what this does.

[enkot]

@pi0 Could you please check? I think it's a really flexible solution.