Detect unneeded `async` keywords on functions (#9966)
## Summary
This change adds a rule to detect functions declared `async` but lacking
any of `await`, `async with`, or `async for`. This resolves #9951.
## Test Plan
This change was tested by following
https://docs.astral.sh/ruff/contributing/#rule-testing-fixtures-and-snapshots
and adding positive and negative cases for each of `await` vs nothing,
`async with` vs `with`, and `async for` vs `for`.
