Fix: consider component and its snippets during css pruning

[7nik]

Fixes #15523

This PR add matching a component against :global(...) in the siblings combinators.
Also, it adds considering the component's content and snippets/slots as potentially located at the root during seeking for sibling elements.

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Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15630-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15630

[dummdidumm]

Code looks good mostly, but the tests need fixing

tests need fixing

[changeset-bot]

🦋 Changeset detected

Latest commit: 5814948

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

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