feat(OnyxTable): implement empty state + define focus outline

[BoppLi]

Relates to #1240

• refactor(OnyxTable): split default slot to distinguish thead and tbody
• feat(OnyxTable): implement empty state + define focus outline

Checklist

• [x] The added / edited code has been documented with JSDoc
• [x] If a new component is added, at least one Playwright screenshot test is added
• [x] A changeset is added with npx changeset add if your changes should be released as npm package (because they affect the library usage)

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 4444e41281365a1b533b54a3d0888001e949db24

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

This PR includes changesets to release 7 packages

| Name | Type | | ------------------------- | ----- | | sit-onyx | Major | | alpha-test-app | Patch | | playground | Patch | | @sit-onyx/chartjs-plugin | Major | | @sit-onyx/nuxt | Major | | @sit-onyx/storybook-utils | Major | | @sit-onyx/vitepress-theme | Major |

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