Deprecate`logLevel` and introduce an internal `lincheck.debug` system property instead

[ndkoval]

The logLevel property is used internally when debugging Lincheck. It should be separate from the API and is rarely needed, even when developing Lincheck itself. Let's deprecate the logLevel property and introduce the lincheck.debug system property instead. Thus, to print the debug info, one needs to run the process with -Dlincheck.debug=true. This will be an internal property for internal use.

[eupp]

Also, maybe we can just use some existing logging library, like kotlin-logging?

[ndkoval]

We don't need such a library, as this property should enable the debug output for us, the developers.

[ndkoval]

There is a better option: we will keep it as a part of the internal API. Therefore, I'm closing the issue.