Define notification type for messages

src/spectator/formatting/formatter.cr

@@ -78,6 +78,7 @@ module Spectator::Formatting
 
     # Called whenever the example or framework produces a message.
     # This is typically used for logging.
+    # The *notification* will be a `MessageNotification` type of object.
     def message(_notification)
     end
 

src/spectator/formatting/notifications.cr

@@ -17,4 +17,7 @@ module Spectator::Formatting
 
   # Structure containing summarized information from the outcome of the test suite.
   record SummaryNotification, report : Report
+
+  # Structure containing a debug or log message from the test suite.
+  record MessageNotification, message : String
 end