Change evaluation order of decorator-injected initializers

[rbuckton]

Per consensus at the May, 2023 TC39 meeting, we need to change the evaluation order of decorator-injected initializers for both accessor and non-accessor fields. More details on the issue can be found here: tc39/proposal-decorators#508.

In summary, given

function minusTwo({ set }) {
  return {
    set(v) { set.call(this, v - 2); },
    init(v) { return v - 2; },
  };
}
function timesFour({ set }) {
  return {
    set(v) { set.call(this, v * 4); },
    init(v) { return v * 4; }
  };
}

class C {
  @minusTwo @timesFour accessor x = 5;
}

const obj = new C();
console.log(obj.x); // (a)

obj.x = 5;
console.log(obj.x); // (b)

Prior to this change, the value at (a) would be 18 (((5) * 4) - 2), while the value at (b) would be 12 (((5) - 2) * 4). This is because the setters wrap and are thus evaluated from the outside-in, while initializers do not wrap and are applied from the inside-out.

After this change, the values at both (a) and (b) would be twelve, as the initializers would be applied outside-in as well.

[DanielRosenwasser]

We should discuss whether it makes sense to bring this in for 5.1, even if it's post-RC.

[EisenbergEffect]

Thank you for the fast turn around on this!