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`.

ruff.schema.json

@@ -3619,6 +3619,7 @@
         "RUF026",
         "RUF027",
         "RUF028",
+        "RUF029",
         "RUF1",
         "RUF10",
         "RUF100",