fix: preserve old dependencies when updating reaction inside fork

[Rich-Harris]

#17335 contains two separate fixes — this extracts one of them, so that they can be reviewed separately

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[changeset-bot]

🦋 Changeset detected

Latest commit: 5fdb5ab

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@17579

[Rich-Harris]

this LGTM — can't approve because it's 'my' PR but @hmnd did all the work. Merging