feat(ecmascript): Builtin class constructors and field initializers

[aapoalas]

With this silly lump of code, Nova now supports field initializers with default constructors.

Field initializers with derived class ECMAScript constructors do not yet work because the initializers are incorrectly run at the very beginning of the ECMAScript constructor instead of as the last step of the super() call. Fixing this wouldn't be too big of a deal but the simple fix won't fix cases like:

let counter = 0;
class Foo extends Bar {
  field = counter++;
  constructor(counter) {
    super();
    console.log(counter);
  }
}

console.log(new Foo(100).field);

Instead of logging 100 and then 0, this would instead log 100 twice because the field initializer code would incorrectly use the counter parameter. It seems likely that I will need to add proper ECMAScriptConstructorFunction variants after all. Luckily now the variants are at least a little less painful to add.