Implement Node.js test suite with jest

[rudokemper]

Goal

Implement jest to add a basic test suite for Node.js.

What I changed

• Implement jest and babel (to transpile ES6 syntax for jest).
• Set up a testing environment in test.js (with a few workarounds, like mocking p-limit which apparently does not play well with jest)
• Added one e2e integration test to generate an MBTiles file from a local, empty style. (Not the best example as of yet, more to come)
• Added a few unit tests focused on testing valid and invalid API keys for Mapbox and Planet

What I'm not doing here

I only added a few basic tests, so this is by no means a comprehensive suite. More tests to come in the future.