feat: allow generic when creating Piscina

[alexframe]

Add ability to pass Task and Return type when creating Piscina class.

Example usage:

const pool = new Piscina<MyTask, MyResult>();

// result resolves as `MyResult` and the args is expected to be a `MyTask`
const result = await pool.run({ my: 'task' }); 

Resolves #231.

[metcoder95]

Can you add some testing to it?

[alexframe]

@metcoder95 I have added a couple of tests, two of which are annotated with @ts-expect-error to ensure that an error is thrown by typescript when an incorrect type is assigned.

Let me know if there's anything else you think is needed.

[metcoder95]

Thanks for the contribution! LGTM