Enable to skip content reading/materialization

Follow up of https://github.com/eclipse-ee4j/jsonb-api/issues/224#issuecomment-590126204

Would be neat to have a way to skip some elements so instead of materializing it. Concretely it means next will need a skip equivalent, visiting the stream but ignoring the content. GetXxx would fail if called after a skip.

Read the JSON-P streaming API and the linked JSON-B discussion first, since no implementation files or tests are named. Define the skip operation's stream behavior, including that skipped content is not materialized and GetXxx calls after skipping fail; the issue is done when this behavior is specified and covered by tests.

Written by the indexing model from the issue text.