perf: validate JSON on skip

[jbee]

First, the examples suite should check if using JsonNode#end() on the root (and thereby skipping the entire input) does also pass the same tests, hence has the same behavior as actually parsing to state. If that is given the checking in the actual parsing could be removed if we always do a skip first as it is known afterwards that the input is indeed valid JSON. That should both improve performance and maintainability as less code is needed.