CSS Selectors can not be written as ordered lists

One of the limitations of JSON-LD 1.0 is that it does not allow ordered nested lists. Therefore a selector like this, is **not permitted**:

```json
{
"@context": {
"selector": {
"@id": "https://axrl.org/selector",
"@type": "https://axrl.org/CSSSelector",
"@container": "@list"
}
},
"selector": [
".foo", [".bar", ".baz"]
]
}
```

Allowing this has been approved fir JSON-LD 1.1, which is currently in draft. I want to avoid using draft features. There are two ways to deal with this situation:

1. We abandon the current selector format in favour of a more verbose one
2. We do not declare in the context that the selector has an order, and adopt this once JSON-LD 1.1 is more stable.

I'm going for option 2, since it seems to be the least impactful. I'm going to keep this issue open as a record of that decision though, and a reminder to revisit this once JSON-LD 1.1 makes it to Candidate Recommendation.

The result of not defining this as an ordered list is that when expanding / flattening the JSON-LD, the nested array will be flattened (`[['foo', 'bar'], 'baz']` becomes `['foo', 'bar', 'baz']`). In practice, that shouldn't be an issue. Since iframes can't be shadow roots, there shouldn't be any confusion about what to do with that selector.