Efficiently detect when positional placeholders appear in the message template

[nblumhardt]

Here, we want to check if a positional template is being used so that the Parameters collection can be included:

https://github.com/datalust/nlog-targets-seq/blob/dev/src/NLog.Targets.Seq/LogEventInfoFormatter.cs#L134

@304NotModified any suggestions here? It seems like it's basically the same API requirement as #2 - get access to the parsed message template.

I imagine this might be as simple as just re-parsing the template in the target when required.