yaz-marcdump: add `jsonlines` output format

Currently, no separator is produced for a collection of JSON records which means the output is not parseable with tools like `yq` and others. Add a new JSON flavor to remedy this:

* `jsonlines`: each JSON record printed on a single line, using `\n` as a record separator

See:
* https://jsonlines.org/
* https://github.com/ndjson/ndjson-spec