Spec extension - string slice

The spec doesn't explicitly exclude strings from being sliced

Index expressions are defined as
```
index-expression = expression bracket-specifier / bracket-specifier
```

and `expression` includes current node and raw-string as follows
```
expression =/ function-expression / pipe-expression / raw-string
expression =/ current-node
```

It should be possible then to do the following and not restrict it to JSON array types only

```javascript
search("FOOBAR", '@[1:3]')
// OUTPUTS: 'OO'
```