fix: minify injected CSS in prod mode

[dummdidumm]

When CSS is externalized we rightfully rely on the following tooling chain to properly minify CSS. When we inject the CSS however, that tooling won't be able to do that, so we gotta do it ourselves. This PR brings back most of that logic that existed in Svelte 4. Fixes #13716

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Tests and linting

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[changeset-bot]

🦋 Changeset detected

Latest commit: 1fcc210

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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svelte	Patch

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@14006

commit: 1fcc210

[Conduitry]

Is it reasonable to extract all of this minification code out to its own file, so we're not sprinkling a bunch of rarely used code throughout the style scoping code?

[dummdidumm]

I think that would make things worse, because I would need to recreate some of the logic around pruning that's already there (like "is this selector used").