Move ClassName to core-java-lang (#5796)

Issue: BAEL-2309

core-java-lang/src/main/java/com/baeldung/className/RetrievingClassName.java

@@ -0,0 +1,9 @@
+package com.baeldung.className;
+
+public class RetrievingClassName {
+
+    public class InnerClass {
+
+    }
+
+}