Support newer JavaScript features in CraftScripts

**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