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 `IsEnabled` is fine for this, though maybe not ideal. But better than `IsEnabledFor` :smile: