Update is_ok() and is_err() to be type guards
Update is_ok() and is_err() to be type guards

This will allow code like this:
```
const result = someFunction(); // returns Result<number, string>
if (result.is_err()) {
    // result is Err<string> here;
} else if (result.is_ok()) {
    // result is Ok<number> here;
}
Vincent-Pang/result-class