fix: emit error on wrong placement of the :global block selector

[7nik]

Closes #14906

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[changeset-bot]

🦋 Changeset detected

Latest commit: 2d108a0

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

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svelte	Patch

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15794-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15794

[Rich-Harris]

thank you!