docs: update README banner

[Ocean-OS]

Closes #13952. This is a redo of #13978, whose branch was accidentally deleted.
This updates the banner to reflect the changed motto/slogan/message of Svelte 5. A dark mode version is also included.

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Tests and linting

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

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0b05256

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-15063-svelte.vercel.app/

this is an automated message

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15063

[paoloricciuti]

Looks good, thanks!