Add "High compliancy" mode

[ova2]

Please add "High compliancy" mode as in Babel (checkbox here https://babeljs.io/repl/). Currently this ES6 code

if (true) {
  console.log(typeof value);  // ReferenceError!  
  let value = "blue";         // Temporal Dead Zone
}

is transpiled to

"use strict";

if (true) {
  console.log(typeof value);  // ReferenceError!
  var value = "blue";         // Temporal Dead Zone
}

and does not throw ReferenceError.