Structured Clone Queues Message preview

Currently, only when queue messages are JSON are they previewable in the Cloudflare dashboard.

The default message type in Cloudflare is Structured Clone, which is quite nice. Cloudflare dashboard should collaborate with that default. Anyways, structured clone is available in all browsers too.

Our team is discussing switching to JSON messages in queues solely because the dashboard does not preview structured clone messages. That seems like needless complexity.