Babel 8 compatibility

[nicolo-ribaudo]

Hey, Babel has an e2e test for Ember which makes me realize that this package cannot seamlessly upgrade to Babel 8.

• The first commit updates the code to still use Babel 7, but in a way that will also work with Babel 8
• The second commit updates the test runner to run, when using modern Node.js versions, as native ESM rather than transpiling through Jest. Without this, it won't be possible to upgrade to Babel 8 because it's written in ESM and Jest has problems with it.
• The third commit adds a CI job to make sure that the codebase is compatible with Babel 8.

This PR doesn't upgrade to Babel 8 yet, but it makes sure that when Babel 8 will be stable and you'll be ready for a major release you can simply update the dependency :)