Unable to use wal2json plugin with the demo

I cannot get the demo running with `wal2json` as the output plugin. The message format looks completely different between `wal2json` and `pgoutput`. FWIW, I tried with both the format versions of `wal2json` and also tried it on different PG versions.

It panics since the pglogrepl.Message is Unknown

```
panic: interface conversion: interface is nil, not pglogrepl.Message

goroutine 1 [running]:
github.com/jackc/pglogrepl.Parse({0xc0000f401e, 0xa8, 0xa8})
/Users/viggy28/go/pkg/mod/github.com/jackc/pglogrepl@v0.0.0-20230318140337-5ef673a9d169/message.go:649 +0x2b9
main.main()
/Users/viggy28/Sourcecontrol/github.com/viggy28/pglogrepl/main.go:136 +0xdda
```

Start by reading main.go around line 136 and the pglogrepl.Parse call shown in the panic, then compare the wal2json and pgoutput message formats described in the report. Done means the demo no longer panics when configured with wal2json, with behavior or support boundaries verified on the PostgreSQL versions and both wal2json format versions mentioned.

Written by the indexing model from the issue text.