fix: guard against customElements being unavailable in browser extension contexts

[dummdidumm]

No test because no way to do that

fixes #14739

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[changeset-bot]

🦋 Changeset detected

Latest commit: 646f330

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

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svelte	Patch

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14933-svelte.vercel.app/

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@14933