Ignore stderr of command if Antora's runtime.silent playbook key is true

If Antora is configured to have silent output (i.e., the runtime.silent playbook key has the value true), Collector should ignore (i.e., silence) the stderr stream of the command by default.

If the command fails in this case, the stderr lines should be emitted retroactively. Alternately, we could consider having the stderr lines follow the setting specified by the failure key (ignore, log, or included in thrown error). However, that may not be clear cut, so if necessary, that should be a separate feature request.