Import libxo-0.9.0:
- Add xo_format_is_numeric() with improved logic to decide if format strings are numeric, so json output quotes them
- Convert docs to sphinx/rst
- update tests
Includes fix for PR 221676:
https://github.com/Juniper/libxo/commit/27d3021cc3cc8cfbe9ddee5930cd7a9afea8f68f#diff-5a0d468963477f7daedb8308c219dd80
PR: 221676
MFC after: 5 days