Per the DAP spec, the event 'body' field should contain any additional
data related to the event. I updated the lldb-dap 'statistics' extension
into the terminated event's body like:
```
{
"type": "event",
"seq": 0,
"event": "terminated",
"body": {
"$__lldb_statistics": {...}
}
}
```
This allows us to more uniformly handle event messages.
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