Setting commands on a transaction returns a Promise

We have e.g.:

```
const tx = redis.tx()
tx.lpush("list", string)
tx.expire("list", 60)
await tx.flush()
```

I'm unsure why e.g.`tx.lpush` returns a `Promise` uncesssarily as this is not an async operaion surely? I expected to return the transaction to enable chaining e.g.:

```
await redis.tx()
.lpush("list", string)
.expire("list", 60)
.flush()
```

If i can assist with such improvements, let me know.

Start by tracing redis.tx() and the transaction command methods such as lpush and expire, then compare their return values with flush(). The work is done when command calls support the requested chaining without breaking the existing transaction usage, with tests covering both return behavior and flushing.

Written by the indexing model from the issue text.