[IMPROVED] Report stalled flow control on mirrors/sources (#8633)

A source/mirror stream could stall if its flow control messages weren't
making its way back to the stream being sourced/mirrored. Usually that
happens with missing `$JS.FC.>` import/export. This PR improves
awareness by letting a stalled error show up in stream info and print a
warn log. Either way, this condition will remain until the user fixes
their imports/exports.

Relates to https://github.com/nats-io/nats-server/issues/8630