Swapping array indexes is not supported

This should probably work, but doesn't.

```ndc
let l = [0, 1];
l[0], [1] = l[1], l[0];
```

With the following error:

```
  × Error while parsing source
  ╰─▶ Invalid assignment target
   ╭────
 1 │ l[0], [1] = l[1], l[0]
   · ────┬────
   ·     ╰── here
   ╰────
```

