[validation] Check if internal tag variant default is correctly supported

Validation is currently failing for 
```
/**
 * @variants internal tag='type' default='custom'
 * @non_exhaustive
 */
export type Analyzer =
  | CustomAnalyzer
...
```
Error:
`Property type is missing in type { filter: string[]; tokenizer: string; } but required in type AnalysisCustomAnalyzer`

The validator is probably ignoring the `default='custom'` annotation