Issue #3: Refactoring ILogger

Moved extension method `IsEnabled` to the `ILogger` interface, renamed `WriteCore` to `Write` and changed the return type to void. This will break other repositories that use the ILogger interface, which I will update once I get feedback on the changes here.
I think this should return `false` only if *all* underlying loggers were disabled, no? Looking at the old logic in `WriteCore`, I think that was the case, no? (Please correct me if I am wrong.)