Is your feature request related to a problem? Please describe.
Many new JavaScript features don't work in Rhino JS. This makes it painful to program CraftScripts due to being stuck in the relative dark ages of JavaScript.

Describe the solution you'd like
The ability to use ES2015+ features in CraftScripts.

Additional context
There's a prototype of adding this ability using the Babel transpiler. Adding this would also open up the ability to use TypeScript. https://github.com/EngineHub/WorldEdit/compare/feature/transpile-craftscripts