Add entry point `onlyClasses` and similar

[rweisleder]

Because of a recent discussion, I extract my suggestion from this comment into a separate issue.

It would be great to have onlyClasses() as entry point. Rules created with onlyClasses() should fail

1. if classes that match the that condition, don't match the should condition, and
2. if classes that don't match the that condition, match the should condition.

Same goes for onlyMethods(), onlyFields(), only(ClassesTransformer) etc.

The current alternative is to write rules "twice":

CompositeArchRule.of(
    classes().that().areAnnotatedWith(RestController.class).should().haveSimpleNameEndingWith("RestController")
).and(
    classes().that().haveSimpleNameEndingWith("RestController").should().beAnnotatedWith(RestController.class)
).as("only classes that are annotated with @RestController should have simple name ending with 'RestController'")

// vs.

onlyClasses().that().areAnnotatedWith(RestController.class).should().haveSimpleNameEndingWith("RestController")