Update JavaScript benchmarks to use ECMAScript classes

[smarr]

It seems the time to switch to using the class syntax.

It's established enough, and easily available in Node.js.
For backwards compatibility, I may need to think about generating more compatible version using Babel or similar.

[smarr]

I'll still need a pure JavaScript version though, and ideally, one that is compatible with ECMAScript without for-of and classes.

Latest code is here: https://github.com/gracelang/moth-benchmarks/tree/master/benchmarks/JavaScriptPure.