feat(ipx): use alternate URL style with format as file extension

[sgarner]

🔗 Linked issue

Resolves #933, #584, #442

❓ 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

Implements (and depends on) the alternate URL style for IPX proposed in unjs/ipx#259

This improves prerendering of images so that the target format is respected in the generated filename, fixing issues with static hosting. For full rationale please see the description in unjs/ipx#259

The original URL style will still be used when the requested format is auto (which is not applicable for prerendering anyway), or when no modifiers are given. Otherwise the new URL style will be used.

I'm not sure whether this needs to be classed as a breaking change. It will cause different URLs to be produced with ipx and ipxStatic providers, but that shouldn't break anything unless users are somehow relying on the old URL style.