feat: support `global.css` file

[larsrickert]

Relates to #1266

Reviewer question: Should we import it by default for Nuxt or also make it opt-in?

Support global.css file that can be optionally imported by a project to apply global application styles. Will be imported by default for Nuxt projects.

Checklist

• [x] The added / edited code has been documented with JSDoc
• [x] All changes are documented in the documentation app (folder apps/docs)
• [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: 2c8296a36d9f330fd257920fe6b418e8bd451f0e

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 | Minor | | 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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