Extract Class: Implement 'Extract Superclass' refactoring

#KT-11017 In Progress

idea/testData/refactoring/extractSuperclass/privateClass.kt

@@ -0,0 +1,7 @@
+// NAME: X
+// SIBLING:
+class A {
+    private open class B
+
+    private class <caret>C : B()
+}