Support ESM for Node.js

ESM is already supported in Node.js since v12 [[docs]](https://nodejs.org/api/esm.html). However, we're currently missing proper support and documentation as follows:

- Top-level await triggers an error
- `.mjs` files are not being considered for linting
- There are no instructions on how to setup a config for ESM in Node.js
- ...anything else?

@satazor @threequartersjohn thoughts?