Output a --- marker to mark the end of a log message in NonDarwinLogger

Figuring out where one log message ends and the next one starts is a little tricky. A `---` marker helped me a lot to find the boundary between two log messages.

Author: ahoppen

Sources/LSPLogging/NonDarwinLogging.swift

@@ -291,6 +291,7 @@ public struct NonDarwinLogger {
         """
         [\(subsystem):\(category)] \(level) \(dateFormatter.string(from: date))
         \(message().value.string(for: self.privacyLevel))
+        ---
         """
       )
     }