use enableCmdLine against PRSchat.EMCO

https://demonnic.github.io/mdk/current/classes/EMCO.html#emco:enableCmdLine
```
PRSchat.EMCO:enableCmdLine("Chat", function()
send("chat " .. str)
end)
```
instead of the two part process you're using and reaching into PRSchat.EMCO.mc for

Start at the PRSchat.EMCO chat-command setup and compare its current two-part process and direct PRSchat.EMCO.mc access with the documented PRSchat.EMCO:enableCmdLine entry point. Done means the chat command uses enableCmdLine with the shown callback and retains the existing chat behavior.

Written by the indexing model from the issue text.