Add completed method in Pool

Instead of doing every time this:
```
const pool = workerpool.pool(
path.resolve(__dirname, 'worker')
)
const tasks: workerpool.Promise[] = []

for (const entity of entities) {
const task = pool.exec('worker', [entity])
tasks.push(task)
}

await Promise.all(tasks)
await pool.terminate()
```

We can do:

```
const pool = workerpool.pool(
path.resolve(__dirname, 'worker')
)

for (const entity of entities) {
pool.exec('worker', [entity])
}

/* Wait all tasks to complete */
await pool.completed()
await pool.terminate()
```

It would be very handy, if this method is added in the future
For example, threads.js already have [this feature](https://threads.js.org/usage-pool#waiting-for-tasks-to-complete)