Add CompileOptions.warningHandling() to specify how compile time warnings should be handled

[lukaseder]

Currently, when there are compile time warnings, we're throwing a ReflectException. For example, if an annotation processor declares supporting Java 11, but we're processing things with Java 15, there's this exception here:

org.joor.ReflectException: Compilation error: warning: Supported source version 'RELEASE_11' from annotation processor 'org.joor.test.CompileOptionsTest$AProcessor' less than -source '15'

That shouldn't be an exception, but just a log message. We should have:

• CompileOptions.warningHandling() as a way to set the warning level
• WarningHandling.ERROR to treat warnings as errors
• WarningHandling.LOG to log warnings on System.out
• WarningHandling.IGNORE to silently ignore warnings

[lukaseder]

Huh I cannot seem to consistently reproduce this warning. May have been some IDE related artifact? Will investigate again, once this can be reproduced more consistently.