Optional chaining for extensions -- `?::`

It would be great if optional chaining was baked into the proposal:

```javascript
document.querySelector("#myElement")?::let(it => {
  // it's safe to do things with the element here
  setupInteractivity(it);
});
```

Equivalent to:

```javascript
let temp = document.querySelector("#myElement");
if (temp != null) {
    setupInteractivity(temp);
}
```