[lldb][lldb-vscode] Pretty print JSON to log files

This makes anlysing test failures much more easy.

For SendJSON this is simple, just use llvm::format instead.

For GetNextObject/ReadJSON it's a bit more tricky.
* Print the "Content-Length:" line in ReadJSON, but not the json.
* Back in GetNextObject, if the JSON doesn't parse, it'll be
  printed as a normal string along with an error message.
* If we didn't error before, we have a JSON value so we pretty print it.
* Finally, if it isn't an object we'll log an error for that,
  not including the JSON.

Before:
```
<--
Content-Length: 81

{"command":"disconnect","request_seq":5,"seq":0,"success":true,"type":"response"}
```

After:
```
<--
Content-Length: 81

{
  "command": "disconnect",
  "request_seq": 5,
  "seq": 0,
  "success": true,
  "type": "response"
}
```

There appear to be some responses that include strings that are themselves JSON,
and this won't pretty print those but I think it's still worth doing.

Reviewed By: wallace

Differential Revision: https://reviews.llvm.org/D156979

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