fix: avoid duplicating escaped characters when parsing styles

[teemingc]

Unblocks sveltejs/kit#15153 (comment)

This PR ensures that escaped characters are handled correctly when storing the value of a CSS declaration. Previously, it was duplicating it.

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🦋 Changeset detected

Latest commit: 352785b

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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Playground

pnpm add https://pkg.pr.new/svelte@17554

[Rich-Harris]

we could early-continue from the first two branches and keep the value += char where it is, it might be a tiny bit cleaner. but LGTM either way