CesiumJS now allows usage of ES2020

Hi all,

Recently we upgraded Cesium’s ESLint configuration to enable usage of ECMAScript 2020. This brings us forward from ES2015, which was the previously supported language target. This upgrade enables us to take advantage of the latest language features in our codebase, such as async/await, BigInt, optional chaining etc. Since linting is built into our pre-commit hook, you’ll instantly be notified if you use any disallowed language features.

If you’re working with the code and have any feedback, please let us know.