Come up with eslint rule that throws if function is labeled as async but doesn't use await

[TimDaub]

• e.g. we use async here but we don't use await in the function's body: https://github.com/neume-network/extraction-worker/blob/cafba34b36422afb488f9b43af8934d8363ea655/src/worker.mjs#L47
• So we should have an eslint everywhere to catch this kind of problem

/cc @il3ven because you had set up neume-network/strategies#78