Extract Function: Add support of initializer expressions

idea/testData/refactoring/extractFunction/initializers/classes/classInitializer.kt

@@ -0,0 +1,6 @@
+// PARAM_TYPES: kotlin.Int
+class A(val a: Int, b: Int) {
+    {
+        println(<selection>a + b</selection> - 1)
+    }
+}