Upgrade to Babel 7.8.0 #116

"Babel 7.8.0 supports the new ECMAScript 2020 features by default: you don't need to enable individual plugins for nullish coalescing (??), optional chaining (?.) and dynamic import() anymore with preset-env." - https://babeljs.io/blog/2020/01/11/7.8.0

We should update zero to latest babel and add an integration test for these features. I think the right change will be here.