[BUGFIX release] Don't setup mandatory setters on array indexes

[pzuraq]

It turns out that removing items from an array (e.g. via splice) will
also remove their descriptors, if any. Could be a browser bug, but I'm
not sure we intended to add setters directly to arrays in the first
place, and if we can't definitely manage them we probably shouldn't.

Fixes #18739

[rwjblue]

After discussion with @krisselden, he suggested we change to using something more like:

function isElementKey(key) {
    return typeof key === 'number' ? isPositiveInt(key) : isStringInt(key);
}

function isStringInt(str) {
    let num = parseInt(str, 10);
    return isPositiveInt(num) && str === '' + num;
}

function isPositiveInt(num) {
    return num >= 0 && num % 1 === 0;
}

// this would replace the conditional
if (isElementKey(keyName)) {
  return;
}

[rwjblue]

@pzuraq ping 😺