Feature: Implement [Symbol.asyncIterator] for .cursor() without a function.

Implementation of `{ [Symbol.asyncIterator]: ... }` when calling cursor without a callback function in order to support _for-await-of_ syntax

See Comment by @porsager in https://github.com/porsager/postgres/issues/1#issuecomment-581777536