[display] DisplayLocal vs FunctionArgument.opt

```haxe
class Main {
static function main() {}

function foo(?bar:Int) {}
}
```

There's no way to tell whether the argument `bar` is actually optional or just `Null` from the data reported by hoveing over it (a `DisplayLocal`).


This is inconsistent with the data obtained from hovering over the function itself (`foo`), which includes this via the `opt` flag.