Extract Function: Add support of initializer expressions

idea/testData/refactoring/extractFunction/initializers/accessors/memberProperty.kt

@@ -0,0 +1,3 @@
+class A(val a: Int, val b: Int) {
+    val foo: Int get() = <selection>a + b</selection> - 1
+}

idea/testData/refactoring/extractFunction/initializers/accessors/memberProperty.kt.after

@@ -0,0 +1,7 @@
+class A(val a: Int, val b: Int) {
+    val foo: Int get() = i() - 1
+
+    fun i(): Int {
+        return a + b
+    }
+}

idea/testData/refactoring/extractFunction/initializers/accessors/topLevelProperty.kt

@@ -0,0 +1,3 @@
+val a = 1
+val b = 1
+val foo: Int get() = <selection>a + b</selection> - 1

idea/testData/refactoring/extractFunction/initializers/accessors/topLevelProperty.kt.after

@@ -0,0 +1,7 @@
+val a = 1
+val b = 1
+val foo: Int get() = i() - 1
+
+fun i(): Int {
+    return a + b
+}