fix: allow double hyphen css selector names

[paoloricciuti]

Closes #15354

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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-15384-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15384

[baseballyama]

Thank you!