fix(vite): don't warm up css deps and normalise urls correctly

[danielroe]

🔗 Linked issue

resolves #20533
resolves #14898

❓ 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

It seems we are over-warming up vite, and in particular warming up lots of Sass files has a significant performance cost in some projects, which is not actually needed.

This PR ensures we align our warmup logic with the upcoming warmup option in Vite 5 (vitejs/vite#14291).

This PR skips warming up CSS dependencies as these are duplicate (ie. they are already included in the output of the first CSS dep). It also applies same logic for normalising URLs as Vite does itself, e.g.:

https://github.com/vitejs/vite/blob/e4c801c552edc4a60659720b89777e29eb93db6b/packages/vite/src/node/plugins/importAnalysis.ts#L604

📝 Checklist

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

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