Fix await-outside-async to allow await at the top-level scope of a notebook

[harupy]

Summary

Fix await-outside-async to allow await at the top-level scope of a notebook.

# foo.ipynb

await asyncio.sleep(1)  # should be allowed

Test Plan

A unit test

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[charliermarsh]

Thanks!