Replace deprecated request module with Node's built-in fetch function

[danielrozenberg]

This uses node-fetch so that we can mock it in tests (nock can't mock the built-in fetch object, see nock/nock#2397), but node-fetch is a thin wrapper around the built-in fetch when it's available, so this is basically like using the built-in one :)

Breakdown of changes:

• Replace request with node-fetch
• Restore the tests that were disabled in Convert to ES Modules #144, use nock to mock network requests instead of stubbing the function
• Convert lots of tests to async functions