Extract Function: Add support of initializer expressions

idea/testData/refactoring/extractFunction/initializers/functions/topLevelFunction.kt.after

@@ -0,0 +1,7 @@
+// PARAM_TYPES: kotlin.Int
+// PARAM_TYPES: kotlin.Int
+fun foo(a: Int, b: Int) = i(a, b) - 1
+
+fun i(a: Int, b: Int): Int {
+    return a + b
+}