Variable scope issue

When using `if` without brackets, the scope stays the same and variables can be shadowed:

```haxe
var expr = '
var a = "test";
if (true) var a = 42;
return a;
';

var parser = new hscript.Parser();
var ast = parser.parseString(expr);
var interp = new hscript.Interp();
trace(interp.execute(ast)); // 42
```

Works as expected when using brackets.