Character.isLetter and .isNumber are not supported

The code I'm porting with Skip fails here:

```
private extension Character {
    var isAlphaNum: Bool {
        isLetter || isNumber
    }
}
```

> This API is not yet available in Skip. Consider placing it within a #if !SKIP block. You can file an issue against the owning library at https://github.com/skiptools, or see the library README for information on adding support