doc: update js-native-api example

Update example that shows how to separate N-API code which is not
Node.js-specific from code which defines a Node.js N-API addon. In its
existing state the example uses the pattern

```C
assert(napi_*() == napi_ok);
```

However, this would result in no N-API calls when building with
`-DNDEBUG`.

This change moves away from assert and uses a macro `NAPI_CALL()` which
throws the string corresponding to the non-`napi_ok` status as a JS
exception and short-circuits the binding by returning `NULL`.

PR-URL: https://github.com/nodejs/node/pull/28657


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>