Intentions: Implement "Convert sealed class to enum" intention

#KT-14245 Fixed

idea/testData/intentions/convertSealedClassToEnum/withNonObjectInheritors.kt

@@ -0,0 +1,7 @@
+// SHOULD_FAIL_WITH: All inheritors must be nested objects of the class itself and may not inherit from other classes or interfaces. Following problems are found: class <b><code>A</code></b>, class <b><code>B</code></b>
+
+sealed class <caret>X {
+    class A : X()
+}
+
+class B : X()