False positive `missing_placeholder_with_metadata ` when it's followed by a key with `@@x-`

When a property with a placeholder is followed by a key prefixed with `@@x-`, a warning `The placeholder is defined in the metadata, but not in the message.(missing_placeholder_with_metadata)` is popped.

i.e:
```json
{
  "some": "some {title}",
  "@some": {
    "placeholders": {
      "title": {
        "type": "String"
      }
    }
  },
  "@@x-comment": ""
}
```