Chained assignment should be an error/warning

At the moment, we can write
```nu
$env.foo = $env.bar = "baz"
```

and even 
```nu
let $foo = let $bar = "baz"
```

without any warning, but the resulting values are
```nu
$env.foo == null
$env.bar == "baz"
```

which is most likely not what was expected.

I think this should be reported as a warning (or even an error if there are no actual use cases for this syntax).