Create subclass intention implemented #KT-8473 Fixed

Can be used for implementing interfaces / abstract classes / sealed classes or extending open classes

idea/testData/quickfix/implement/privateNested.kt

@@ -0,0 +1,10 @@
+// "Create subclass" "true"
+// SHOULD_BE_AVAILABLE_AFTER_EXECUTION
+
+class Container {
+    private open class <caret>Base {
+        open var x: String = ""
+
+        open fun foo(): String = ""
+    }
+}