`problem.yaml:type`: Specify that `pass-fail` is implied when neither `pass-fail` or `scoring` is provided

[mpsijm]

https://icpc.io/problem-package-format/spec/2023-07-draft.html#type currently says that pass-fail is "Default", but it does not explicitly say that type: interactive implies type: [pass-fail, interactive]. However, DOMjudge interpreted the mutual-exclusive constraint as "at least one should be provided": https://github.com/DOMjudge/domjudge/pull/2979/files#r2055970154

Previous discussion on this topic in e.g. #131 and #378 appear to agree on the interpretation that pass-fail is implied, and BAPCtools has the same interpretation.