fix: allow classes to be reassigned

[Rich-Harris]

Ran into this while upgrading a site to Svelte 5. There were other issues besides this one — for some reason files inside SvelteKit were being passed by vite-plugin-svelte to Svelte, incorrectly, which meant this line triggered the bug — but this is as good a place to start as any.

No test because it doesn't really seem worth it, doesn't seem like something we'll regress on

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[changeset-bot]

🦋 Changeset detected

Latest commit: 40cee2e

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

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