fix build failure for TS consumers

[amitlevy21]

Before this commit, Typescript consumers would get the following error:
"This module can only be referenced with ECMAScript imports/exports by turning on the 'esModuleInterop' flag and referencing its default export"
consumers of this library would have to set esModuleInterop to true in
their tsconfig.json in order to build with this library.

With this commit, consumers can simply import without adding
esModuleInterop to their configuration.

Examples:

// ES6:
import { PromisePool } from '@supercharge/promise-pool'

// ES5:
const { PromisePool } = require('@supercharge/promise-pool')

Closes: #38

@marcuspoehls Let me know what you think, Thank you!

[marcuspoehls]

@amitlevy21 Thank you Amit! Looks good 👍 I’ll publish these changes on NPM as a new major release. It requires users to update their imports of the promise pool (as you did in the tests).

[amitlevy21]

@marcuspoehls Great! Happy to help 🚀